Sensor with improved drift stability

ABSTRACT

The present invention provides an optical fluorescence based sensor for measuring the concentration of a gas (e.g., CO 2  or ammonia) in a medium such as blood which has improved drift stability. In a preferred embodiment, the sensors of the present invention comprise microcompartments of an aqueous phase having a pH sensitive sensing component within a hydrophobic barrier phase. The sensors of the present invention are substantially free of partitioning species other than the analyte of interest which can migrate from one phase to the other in response to a change in pH in the aqueous phase and which substantially affect the concentration dependent signal. In an alternative embodiment, the sensors of the present invention are constructed so as to retard the migration of partitioning species, thus reducing the initial rate of drift.

FIELD OF THE INVENTION

The present invention relates generally to sensors for measuring theconcentration of an analyte of interest in a medium. In particular, thepresent invention includes a sensor for the monitoring of blood gas(e.g., ionized hydrogen and carbon dioxide) concentrations.

BACKGROUND OF THE INVENTION

The present invention relates to sensing or determining theconcentration of an analyte of interest in a medium. More particularly,the invention relates to sensor apparatus or systems and methods forsensing the concentration of an analyte of interest, for example, carbondioxide, in a medium, for example, blood. The invention also relates tosensor apparatus or systems and methods for sensing the concentration ofan analyte of interest, for example, ammonia, SO₂, or NO₂ in industrialsettings and environments.

It is sometimes necessary or desirable for a physician to determine theconcentration of certain gases, e.g., oxygen, ionized hydrogen andcarbon dioxide, in blood. This can be accomplished utilizing an opticalsensor which contains an optical indicator responsive to the componentor analyte of interest. The optical sensor is exposed to the blood, andexcitation light is provided to the sensor so that the optical indicatorcan provide an optical signal indicative of a characteristic of theanalyte of interest. For example, the optical indicator may fluoresceand provide a fluorescent optical signal as described in Lubbers et at.U.S. Pat. No. Re 31,897 or it may function on the principles of lightabsorbance as described, for example, in Fostick U.S. Pat. No.4,041,932.

The use of optical fibers has been suggested as part of such sensorsystems. The optical indicator is placed at the end of an optical fiberwhich is placed in contact with the medium to be analyzed. This approachhas many advantages, particularly when it is desired to determine aconcentration of analyte in a medium inside a patient's body. Theoptical fiber/indicator combination can be made sufficiently small insize to easily enter and remain in the cardiovascular system of thepatient. Consistent and accurate concentration determinations areobtained.

Optical fluorescence CO₂ sensors commonly utilize an indirect method ofsensing based on the hydration of CO₂ to carbonic acid within anoptionally buffered aqueous compartment containing a pH sensitive dye.The aqueous compartment is encapsulated in a barrier material which isimpermeable to hydrogen ion but permeable to CO₂. An opticallyinterrogated pH change in the internal aqueous compartment can then berelated to the partial pressure of CO₂ in the monitored sample. Ionicisolation of the internal aqueous phase may be achieved by directlydispersing aqueous droplets throughout the isolating matrix as describedin U.S. Pat. No. 4,824,789 (Yafuso et al.) which is herein incorporatedby reference. Alternatively, the aqueous phase may be sorbed into porousparticles which are then dispersed throughout the isolating matrix asdescribed in U.S. Pat. No. 4,557,900 (Heitzmann). The isolation matrixor "barrier" is typically a crosslinked silicone polymer. Such sensingchemistries may further comprise aqueous and silane viscosifiers anddispersing aids to stabilize the dispersion prior to crosslinking thesilicone polymer.

Unfortunately, a characteristic feature of these types of sensors isreversible "CO₂ conditioning drift" (hereinafter for brevity sometimesreferred to as "drift"), a response instability accentuated by largechanges in CO₂ partial pressure. FIGS. 3a and 3b show typical drift datafor a CO₂ sensor formulation in an ex-vivo blood gas sensing system.FIG. 3a illustrates a typical "intensity" plot, showing the intensity ofa dye (e.g., the fluorescence intensity) as a function of time after astep change from a medium equilibrated with air (pCO₂ =0.2 mm Hg) to amedium equilibrated with 6 volume percent CO₂ (pCO₂ =45.6 mm Hg). FIG.3b illustrates a plot of "measured" CO₂ concentration (calculated basedon a calibration curve correlating CO₂ concentration or partial pressureto either dye intensity or a parameter such as a ratio of dyeintensities) as a function of time after a step change from a mediumequilibrated with air (pCO₂ =0.2 mm) to a medium equilibrated with 6volume percent CO₂ (pCO₂ =45.6 mm). If a sensor in thermodynamicequilibrium with air equilibrated buffer (pCO₂ ≅0.2 mmHg) is suddenlyexposed to an elevated CO₂ level such as a physiological CO₂ level (pCO₂≅45.6 mm), the measured fluorescence intensity (illustrated in FIG. 3a)will change to a new reading within 1-2 minutes. However, when thesensor is maintained at this elevated CO₂ condition (e.g., pCO₂ ≳40 mm)for several hours, the measured intensity drifts asymptotically in adirection generally opposite to the initial fast response. This isreferred to as " CO₂ conditioning drift." The initial response toelevated CO₂ may be partially or completely regained if the sensor is"deconditioned" for several hours at the baseline state, therefore, theCO₂ conditioning effect is reversible.

This drift instability has been recognized in this type of sensor,although the specific drift mechanism has been in debate. For example,"nonspecific drift" is referred to in U.S. Pat. Nos. 5,246,859 and4,943,364, and characterized as an instability upon exposure to low orhigh levels of CO₂. No mechanism causing this drift was postulated inthese patents. One proposal put forth to explain CO₂ conditioning driftis outlined by Otto Wolfbeis in "Fiber Optic Chemical Sensors andBiosensors", Vol. 2, Chap. 11-V. Specifically, CO₂ conditioning drift isattributed to a reversible migration of water in and out of the aqueousindicator compartment, driven by a CO₂ dependent mismatch of theosmolarities for the aqueous indicator phase and the external mediumbeing sensed. The CO₂ conditioning drift has thus been attributed to thechange in the pH/pCO₂ relationship for the internal aqueous indicatorphase as the indicator and buffer concentrations change.

Koch et al., U.S. Pat. No. 4,943,364, disclose a CO₂ sensor whichpurports to have minimal drift comprising: a hydrolyzed dye/gel polymer;optionally a solution permeable membrane; and a gas-permeable membrane.Koch et at. postulate that the cause of sensor drift in their system isdue to the gradual loss of weakly bonded dye molecules from the dye/gelpolymer structure. To lessen this problem Koch et al. treat theirdye/gel polymer with base to remove weakly bonded dye molecules from thepolymer. Unfortunately, sensors of the type described in Koch et at. areexpensive to manufacture and difficult to uniformly produce.

Nelson et at., U.S. Pat. No. 5,246,859 discloses a carbon dioxide sensorand method for making carbon dioxide sensors comprising a bicarbonatebuffer solution having a concentration of at least 100 mM, a hydroxypyrene trisulfonic acid pH indicator, and a polyvinylpyrolidone aqueousphase viscosifier. The sensor may be optionally exposed to carbondioxide gas (between 2 and 100 weight percent) prior to use. The sensorpurports not to exhibit non-specific drift.

It would be desirable to provide a sensor which has a fast responsetime, is free of CO₂ conditioning drift, and is easily manufactured.

RELATED APPLICATIONS

Of related interest is U.S. patent application, filed on even dateherewith by the assignee of this invention: "Sensing Elements andMethods for Making Same", Ser. No. 08/159,799; and copending U.S. patentapplications Ser. No. 08/136,967 "Emission Quenching Sensors" and08/137,289 "Sensors and Methods for Sensing" which are hereinincorporated by reference.

SUMMARY OF THE INVENTION

We have discovered an optical fluorescence based sensor (e.g., formeasuring CO₂ concentration in a medium such as blood) with improveddrift stability. For sensors comprised of a pH indicator containing anaqueous phase encapsulated in a non-polar, CO₂ permeable barriermaterial, we have discovered that adventitious pH-titratablepartitioning species other than the analyte of interest can reversiblymigrate between the indicator and barrier phases as a function of pH,and therefore as a function of CO₂ partial pressure. We have alsodiscovered that these species can also irreversibly migrate (or "leach")from the sensor to the medium. The migration of the partitioning speciesgenerates a pH response which can substantially affect the analyteconcentration dependent signal. By careful choice and/or purification ofsensor materials and components, we have minimized the presence of theseadventitious species and developed a substantially drift-free sensorformulation. Alternatively, by proper choice of buffer composition andindicator pKa, we has discovered compositions which minimize the CO₂dependent migration of adventitious species, further stabilizing thesesensors.

We have made the unexpected discovery that one cause of the driftinstability (commonly referred to as "analyte conditioning drift" ormore specifically as "CO₂ conditioning drift") is actually a pHhysteresis phenomenon resulting from the presence of pH-titratablematerials (hereinafter referred to as "partitioning species") whichmigrate in and out of the aqueous indicator phase as a function ofinternal pH, and therefore as a function of CO₂ partial pressure. Whileionized species are essentially insoluble in non-polar barrier materialssuch as silicone, the neutral form of organic acids or bases can be verysoluble in both the aqueous and silicone phases. For example, if an airequilibrated sensor comprising a sodium acetate impurity in the aqueousindicator phase is suddenly exposed to an elevated CO₂ level, such thatthe internal compartment pH drops from pH 9 to pH 7, a sudden 100 foldincrease in acetic acid concentration occurs creating a thermodynamicdriving force for slow migration of charge neutral acid into thesilicone. Acid depletion from the aqueous indicator phase then inducesadditional CO₂ uptake and further protonation of acetate ions. Thisdisplacement of acetate by sodium bicarbonate changes the pH/pCO₂relationship for the aqueous indicator phase, resulting in a slow risein pH in opposition to the initially imposed pH drop. The migration or"partitioning" process continues until the equilibrium partitioningratio for acetic acid is reestablished between the aqueous indicatorphase and the silicone. Upon returning to the air equilibrated baselinethe process is reversed. Although not recognizing the cause of the CO₂drift problem, it has been a customary practice in the sensor field topackage continuous blood gas monitoring sensors in a "CO₂ conditionedstate" (i.e., at elevated CO₂ levels) thereby somewhat reducingexcessive drift upon initial exposure to physiological CO₂ levels. Withour new formulation, this practice should no longer be required.

A corresponding hysteresis process is operative for organic bases suchas amines, since they can also migrate from the silicone phase into theaqueous indicator phase when the CO₂ level is elevated. For example, ifan air equilibrated sensor comprising an amine impurity in the siliconephase is suddenly exposed to an elevated CO₂ level, such that theinternal compartment pH drops from pH 9 to pH 7, a sudden 100 foldincrease in ammonium ion concentration occurs creating a thermodynamicdriving force for slow migration of charge neutral amine from thesilicone into the aqueous phase. Amine uptake to the aqueous indicatorphase then induces additional CO₂ uptake and further protonation ofamine. This accumulation of ammonium ions changes the pH/pCO₂relationship for the aqueous indicator phase, resulting in a slow risein pH in opposition to the initially imposed pH drop. The migration or"partitioning" process continues until the equilibrium partitioningratio for amine is reestablished. Upon returning to the air equilibratedbaseline the process is reversed. Notably, the effect on the sensorresponse is identical for both acetate depletion and amine uptake; thatis, when moved from an air equilibrated medium to a medium equilibratedat a higher CO₂ level, a negative mm drift will result in the presenceof either type of species. This hysteresis process can also occur forthe indicator dye itself if the dye exists in equilibrium with apartitionable charge neutral form.

Another cause of drift instability (referred to as "saline conditioningdrift" or "saline drift") is actually a pH dependent phenomenonresulting from the presence of pH-titratable materials which migratefrom the sensor to the medium (e.g., typically an aqueous buffersolution which has a pH of 9 when at air equilibrium) as a function ofexternal pH, and therefore as a function of CO₂ partial pressure. Forexample, if an air equilibrated sensor comprising an amine impurity inthe silicone is suddenly exposed to an elevated CO₂ level, the pH of theexternal buffer drops from pH 9 to pH 7, creating a driving force forslow migration of charge neutral amine from the silicone into theexternal aqueous medium where it becomes protonated. This processimpacts the amount of amine available for partitioning into the internalaqueous compartment. Notably, the effect on the sensor response isopposite to the previously discussed CO₂ conditioning drift. That is tosay, the sensor exhibits a positive mm drift due to these migratingspecies.

With this new understanding, we have developed new CO₂ sensorformulations which contain less than a critical amount of titratablepartitioning species, and exhibit substantially drift free response. Asused herein the terms "drift free" or "substantially drift free" meanthat the sensor provides a signal which drifts less than 12% (i.e., <5.5mm) over a three hour period when moved from a medium equilibrated witha gas having a pCO₂ of 0.25 mm to a medium equilibrated with a gashaving a pCO₂ of 45.6 mm as herein described (see Example 1 and FIG. 3bfor details of this calculation). More preferably, the sensor provides asignal which drifts less than 6% when moved from a medium equilibratedwith a gas having a pCO₂ of 0.25 mm (i.e., air equilibrated) to a mediumequilibrated with a gas having a pCO₂ of 45.6 mm. Most preferably, thesensor provides a signal which drifts less than 3% when moved from amedium equilibrated with a gas having a pCO₂ of 0.25 mm to a mediumequilibrated with a gas having a pCO₂ of 45.6 mm. This advance offersseveral practical advantages.

In one embodiment, the sensor is maintained in an air-equilibratedbuffer condition, except during temporal physiological CO₂ sampling. Forexample, certain commercial ex-vivo sensing systems contain a sensingelement (e.g., a sensor composition housed in a cassette which isattached to an optical fiber or which alternatively contains anexcitation light source) in the a-line circuit of arteriallycatheterized patients. Blood gas levels are monitored temporally (e.g.,"on demand" by the health care provider or automatically by means of aprogrammed pump or motor) by drawing blood up the saline drip line intothe sensor cassette, and then allowing the blood to return to thepatient. The present invention, when used in such a configuration,should remove earlier limitations on the frequency and duration ofphysiological CO₂ sampling that can be achieved without incurring orinducing drift.

In another embodiment, this invention provides a new and improved gassensor of such a size so as to be capable of being introduced directlyinto the body of a patient as for instance by intravenous, intraarterialor interstitial introduction. The sensor may be stored in an airequilibrated buffer medium prior to use without incurring or inducingdrift. The sensor can provide a "continuous" signal representative ofthe gas concentration over a prolonged period and over wide ranges ofanalyte concentration without incurring or inducing drift.

Sensors of the present invention are easily calibrated and may reside inthe calibration medium before, during and after calibration without thespecial precautions commonly employed to limit "CO₂ conditioning drift"or "saline drift".

In general, the magnitude of drift exhibited by traditional sensors isproportional with the buffering capacity of the aqueous indicatorcompartment. Sensors of the present invention, preferably beingsubstantially free of partitioning species, enjoy the advantage of beingfar less susceptible to drift even when the buffering capacity of theaqueous indicator phase is decreased. This enables design of sensorswith a faster response time, without introducing a prohibitive rateand/or amount of drift. In addition, the gas sensors of the inventionare stable (i.e., drift free), reproducible and tolerant of productionvariables without detracting from the inherent properties of the gassensors. Alternatively, one can reduce the drift exhibited by sensorsthat contain small amounts of partitioning species by employing higherbuffer concentrations as herein described. Finally, one may combine ahigher buffer concentration with a "clean" sensor chemistry and producea drift free sensor which is less susceptible to externally induceddrift such as might be caused by exposure of the sensor to a mediumwhich contains a partitioning species.

This can be advantageously accomplished in a gas sensor which comprisesan aqueous first phase including a dye and a hydrophobic second phase,and which is substantially free of partitioning impurities which canmigrate from one phase to the other in response to a change in pH in thefirst phase and which substantially affect the concentration dependentsignal. In a preferred embodiment, the first phase comprises an aqueousbuffered solvent and a dye and the second phase comprises a cross-linkedpolymeric material which is gas permeable, light permeable and isimpermeable to liquid water and protons.

In one embodiment, the first phase comprises a dye in a hydrophilicfluid which is absorbed or adsorbed on carrier beads or particles. Thesebeads or particles are then taken up in a matrix of a hydrophobicmaterial. For example, an aqueous solution of a pH responsive dye (e.g.,8-hydroxypyrene-1,3,6-trisulfonic acid "HPTS") in sodium bicarbonate isabsorbed into the voids of polyacrylamide particles (e.g., having adiameter of approximately 35 μm). The particles are then incorporatedinto a polymeric precursor which is then cured to form a disk ofpolymeric material (e.g., polydimethyl siloxane) which is incorporatedinto a cassette fixture.

In another presently preferred embodiment, the first and second phasesare formed into a permanent "emulsoid" of suspended or dispersedmicro-compartments of the aqueous first phase in the cross-linkedpolymeric second phase wherein the micro-compartments of the aqueousfirst phase are preferably smaller than 5 microns, and more preferablysmaller than 2 microns. In yet an alternative embodiment, the firstphase and second phase may be provided as a laminate.

In an illustrative embodiment of the invention, the dye is a pHsensitive dye, the aqueous buffer solvent is a physiological pH rangebuffer solution as for instance a bicarbonate ion based buffer solution.In this illustrative embodiment the polymeric material is a siliconematerial as for instance a siloxane material which is carbon dioxidepermeable. More specifically the material is polydimethylsiloxane orpolydimethylsiloxane copolymers. The dye in the illustrative embodimentis the trisodium salt of hydroxypyrene trisulfonic acid ("HPTS").

In an illustrative embodiment of the process, the dye is present in theaqueous phase in a concentration of about 1 to about 15 millimolar andthe buffer is present in the aqueous phase in a concentration of fromabout 1 to about 100 millimolar. More preferably, the dye and buffer arepresent in a concentration of from about 1 to about 10 and 1 to 50millimolar, respectfully, and most preferably the dye and buffer arepresent in a concentration of from about 1 to 5 and 5 to 20 millimolar,respectfully.

A further advantageous process of preparing a gas sensor comprisesdissolving a quantity of a dye in a quantity of aqueous buffer solutionfollowed by vigorously mixing the buffer solution with a quantity of apolymeric precursor of a cross-linked polymeric material so as to forman emulsion (or suspension) of the buffer solution and the polymericprecursor. Then a quantity of a cross-linking agent and catalyst areadded to the emulsion. The catalyzed emulsion is formed into a shape(e.g., a small drop at the end of an optical fiber or a sheet or discsuitable for insertion into a sensor cassette) and cured to form apermanent emulsoid (which is substantially free of partitioning specieswhich can migrate from one phase to the other in response to a change inpH in the buffer solution and which substantially affect the analyteconcentration dependent signal) of micro-compartments of the dyecontaining aqueous buffer solution in the cross-linked polymericmaterial. The above process can be augmented by adding a quantity of anemulsification enhancement agent (e.g., a thickener or surfactant) tothe solution of the dye in the buffer so as to form a mixture of the dyeand the emulsification enhancement agent in the buffer and/or by addinga dispersing agent such as fumed silica to the hydrophobic phase.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be better understood when taken in conjunction withthe drawings wherein:

FIG. 1 is an elevational view in section of a droplet of materialutilized in the preparation of a gas sensor of the invention;

FIGS. 2a, 2b, 2c, and 2d are views in section of a gas sensor of theinvention;

FIGS. 3a and 3b are plots illustrating CO₂ conditioning drift for aconventional sensor, the plots have been annotated to help describe twoalternative methods of quantifying the drift;

FIGS. 4a and 4b are comparison plots of two CO₂ sensors showing theimproved drift stability of a sensor of the present invention comparedto a conventional sensor;

FIG. 5 is a view in section of a gas sensor of the present inventionwhich comprises a flow through cassette comprising a sensing element;

FIG. 6 is a view in section of a gas sensor of the present inventionwhich comprises an asymmetric porous barrier film with an aqueous phaseentrapped within the pores of the film;

FIGS. 7 and 8 are two views in section of a gas sensor of the presentinvention which comprise a flow through cassette comprising a preformedlaminate sheet sensing element;

FIG. 9 is a graphical representation of CO₂ and saline drift for aconventional sensor; and

FIGS. 10a-g are contour plots of predicted sensor performance based on amathematical treatment of the partitioning process.

This invention utilizes certain principles and/or concepts as are setforth in the claims appended to this specification. Those skilled in thegas sensing arts to which this invention pertains will realize thatthese principles and/or concepts are capable of being illustrated in avariety of embodiment which may differ from the exact embodimentsutilized for illustrative purposes in this specification. For thesereasons, the invention described in this specification is not to beconstrued as being limited to only the illustrative embodiments but isonly to be construed in view of the appended claims.

DETAILED DESCRIPTION OF THE INVENTION

As used herein, the terms "aqueous first phase" or "aqueous phase" referto the hydrophilic phase or phases of a multiphase sensor whichcomprises a pH responsive indicator component ("dye") and which morepreferably, but not necessarily, further comprises water.

As used herein, the terms "hydrophobic second phase" or "hydrophobicphase" refer to that phase of a multiphase sensor which separates anaqueous phase (comprising an indicator component) from the mediumcontaining the analyte of interest. As used herein, the terms "polymericphase" or "silicone phase" refer to a hydrophobic second phase whichcomprises a polymer material or silicone material respectfully.

As used herein, the term "partitioning species" refers to those species,other than the analyte of interest, which can migrate from the aqueousphase to the hydrophobic phase (or vice versa) in response to a changein pH in the aqueous phase and which substantially affect theconcentration dependent signal (i.e., the signal provided by theindicator component which is proportional to the concentration of theanalyte of interest in the medium being measured). A material is"substantially free" of partitioning species when the species are nolonger capable of substantially affecting the concentration dependentsignal.

As used herein, the term "emulsion" refers to a uniform multi-phasesystem of two or more liquids and includes dispersions and suspensions.The use of the term emulsion is not limited to thermodynamically stablemixtures or mixtures containing emulsifiers. As used herein, the term"stable emulsion" refers to emulsions which remain substantially uniform(macroscopically) for long enough period of time to allow the emulsionto be formed into the desired configuration, e.g., a period of at leastseveral hours.

As used herein, the term "emulsoid" refers to a multi-phase systemcomprising micro-compartments of a dispersed phase in a second solidphase (e.g., a cross-linked polymer phase).

As used herein, the term "response time" refers to the time necessaryfor the concentration dependent signal of a given sensor to reflect theconcentration of the analyte of interest when the sensor is exposed tothe medium containing the analyte. The response time includes any timenecessary for the sensor to stabilize to the medium, but does notinclude the time over which migration of partitioning species occurs tointroduce drift. Preferred sensors of the present invention have aresponse time less than 5 minutes, more preferably less than 2 minutes,and most preferably less than 1 minute.

As used herein, the term "purify" refers to the removal or substantialremoval, from a material or article, of any partitioning species whichmay be present in the material or article. As used herein, the term"immobilize" refers to the fixing of a partitioning species to anotherspecies or material so as to render the partitioning species incapableof migration.

As used herein, the term "specific sensitivity" refers to thedifferential change in concentration of the basic form of the indicator(I₁ I₂ /I₁) at two CO₂ partial pressures which bracket the range ofinterest (i.e., I₁ is measured when the sensor is exposed to a mediumequilibrated with 2.8 volume % CO₂ and I₂ is measured when the sensor isexposed to a medium equilibrated with 8.4 volume % CO₂).

In one embodiment, this invention is directed to a gas sensor which canbe utilized with a fiber optical cable, i.e., a single optical fiber ora bundle of the same. The fiber optic cable is associated withappropriate optical and electronic devices for imposing an opticalsignal from the gas sensor. A plurality of techniques for transmittingand reading appropriate optical signals can be utilized with the gassensors of the invention. For brevity of this specification, the opticsand electronics for gas sensing will not be reviewed in detail,reference being made to the above referenced patents to Lubbers et al.and Heitzmann. For these reasons the entire disclosures of U.S. Pat. No.RE 31,879 to Lubbers et al. and U.S. Pat. No. 4,557,900 to Heitzmann areherein incorporated by reference. Notably, other means of transmittinglight to and from the sensor may be employed. For example, a lightsource such as a LED may be placed next to or against the sensor.

According to the present invention a dye or optical indicator isutilized for sensing a gas of interest. The dye can be one which actswith the gas of interest either by directly interacting with the gas orby indirectly acting with the gas, as for example, by sensing a pHchange in a medium wherein the pH change is caused by interaction of thegas of interest with that medium. Interaction of the gas of interestwith the dye, either directly or indirectly, can be monitored by anysuitable optical technique as for instance by either fluorescence or byabsorption.

In a presently preferred embodiment, a solution of a suitable indicatordye is formed in an aqueous buffer. The aqueous phase is then emulsifiedwith (or uniformly dispersed or suspended with) a liquid precursor of apolymeric material. During the emulsification or suspension step, theaqueous phase is broken up into very small droplet sizes. The polymericmaterial is chosen such that the aqueous phase is not readily soluble ineither the precursor materials for the polymeric material or thepolymerized polymeric material. Thus the aqueous phase always retainsits integrity. By emulsifying or suspending the aqueous phase into thepolymeric precursor materials, very small discrete "micro-compartments"or cells of the aqueous first phase can be formed in the polymericsecond phase. Upon curing or crosslinking of the polymeric phase, thesemicro-compartments are fixed in dispersed positions which areessentially uniformly scattered throughout the polymeric material. An"emulsoid" of the aqueous first phase is thus formed in the polymericsecond phase. Since the aqueous phase in this preferred embodiment isvery evenly distributed within the polymeric phase, when it is fixed inposition in the emulsoid, its concentration is very evenly distributedthroughout the emulsoid. Because the concentration of the aqueous phaseis uniform through the emulsoid, the sensing characteristics of the gassensor of the invention are also very uniform. Contrary to other gassensors, by using very small emulsion sized particles, the surface areaof the individual micro-compartments and thus the totality of themicro-compartments of the aqueous phase is very large. Gas exchangebetween the polymeric phase and the aqueous phase is across theinterface between the surface of the aqueous phase and the polymericphase. Because the surface area of the aqueous phase which is in contactwith the surface area of the polymeric phase is very large, for the gassensors of this invention, gas exchange to the sensing aqueous phase isfast and is uniformly sensitive to the gas concentration within thepolymeric phase.

We have discovered that certain materials used in traditional CO₂sensors, for example, in the aqueous first phase, in the hydrophobicsecond phase, or in other parts of the sensor, undesirably contribute toCO₂ conditioning drift and/or saline drift. These materials maythemselves contribute to drift or contain "impurities" or residualspecies (hereinafter collectively referred to as "partitioning species")that contribute to the drift problem. The amount of such materials orimpurities needed for drift to occur is extremely small. Because of thelarge number of ingredients and materials that go into a typical CO₂sensor, titratable partitioning species are ubiquitous unlessextraordinary precautions are taken to eliminate them or control them.Each and every part of the sensor must be considered for its potentialcontribution of titratable partitioning species. This includes theaqueous phase (including, for example, thickeners or viscosifiers usedtherein), the hydrophobic phase (including, for example, the siliconepolymer and/or cross-linker, and fillers used therein), any optionalfilms or overcoats (for example, substrate films or webs used whencoating sensing elements, optical barrier films, etc.), and any optionaladhesives and/or adhesion promoters used to secure the sensor to anoptical fiber, cassette, or a substrate film. The partitioning speciesmay also be liberated or released from one or more of the sensorcomponents as a result of a subsequent process (e.g., a heating process)or exposure to an environment. For example, some materials containedwithin traditional sensors contain species which are believed to becomepartitioning species only when the sensor is steam sterilized or heated(e.g., dextran thickeners may liberate carboxylic acids and certainpolyester films, used as a substrate upon which to coat the sensor, mayliberate terepthalic acid). With this new understanding of the causes ofCO₂ conditioning drift and saline drift we have also discovered severalmethods to provide sensors which are essentially drift free. By carefulselection and/or purification of components we have developed CO₂sensing chemistry, and sensing elements, which are essentiallydrift-free, and which are not adversely affected by steam sterilization.

We have also discovered that one can control the rate of drift bychanging sensor geometry. For example, the migration of species betweenthe aqueous phase and hydrophobic phase can be slowed by appropriatebarrier means such as an asymetric nylon membrane. Likewise, appropriatecoatings or barriers between the sensor and the medium can slow themigration of species from the sensor to the medium (i.e., slow the rateof "saline drift").

At a minimum, the aqueous phase must contain an indicator of the gas ofinterest for which the sensor is being used. Other materials can beincorporated into the aqueous phase micro-compartments subject to thelimitation described herein that the other material not contribute toappreciable analyte conditioning drift or saline drift. Depending on thegas of interest, these other materials would be chosen to contribute tothe operating characteristics of the gas sensor. For example, additionalmaterials can be added to promote the emulsification or suspension ofthe aqueous phase into the polymeric phase. Further, they can be addedto lower the vapor pressure of the aqueous phase in the polymeric phaseso as to retard the evaporation of the aqueous phase during formation ofthe gas sensor of interest. Aside from materials which contribute to thephysical formation of the emulsoid of the aqueous phase in the polymericphase, further additives can be added to the aqueous phase forenhancement of the storage and/or operating characteristics of the gassensor as for instance osmoregulatory agents (e.g., NaCl) and/orbacteriostatic agents.

A particular gas of interest for the gas sensor of this invention iscarbon dioxide. For sensing carbon dioxide a pH sensitive dye would besolubilized in the aqueous phase. Gas exchange through the polymericphase and into the aqueous phase solubilizes the carbon dioxide gas inthe aqueous phase as carbonic acid which interacts with the buffer ions.The dye chosen is one which is responsive to the concentrations of theionic species of the carbonic acid in the aqueous phase, i.e., anacid-base responsive dye.

For the purposes of this invention, we consider aqueous compositionswhich can be prepared by addition of various amounts of indicator,sodium bicarbonate, and a 50:50 mix of monobasic sodium phosphate anddibasic sodium phosphate. Those skilled in the art will recognize thatthese same compositions can be prepared in alternate ways withoutaffecting the resultant buffer composition or the resultant bufferperformance as a function of CO₂ partial pressure. Furthermore, it isrecognized that the sodium ion or chloride ion as counterions for thebuffering species can be replaced by other salt forming ions withoutchanging the scope or intent of this invention.

Preferred for use in sensing carbon dioxide is a bicarbonate ion basedbuffer in the aqueous phase. Such a buffer can be chosen so as to have abuffer range compatible with the response range of the dye. Such a rangemight, for instance, mimic the physiological pH range of blood. Suitablefor the preparation of such a bicarbonate ion buffer would be sodiumbicarbonate, sodium carbonate and sodium hydroxide or other suitablebuffer agents. For measuring blood carbon dioxide with hydroxypyrenetrisulfonic acid, a pH range of pH 7.0 to pH 8.0 is the most desirable.

As discussed in Example 9, the concentration of the sodium bicarbonateand HPTS indicator can be chosen to optimize the sensitivity of thesensor over the range of CO₂ partial pressures commonly encounteredduring blood gas monitoring. In addition, this optimized sensitivity canbe obtained at a higher sodium bicarbonate concentration by increasingthe pKa of the indicator. This offers the advantage that for sensorscontaining only amine impurities, the amount of CO₂ conditioning driftcan be reduced.

At high concentrations of sodium bicarbonate, the pH of the aqueousphase is high over the entire operating range of the sensor. When theoperating pH range is sufficiently high relative to the pKa of the amineimpurity, the impurity will be substantially deprotonated at all CO₂partial pressures and will preferentially reside in the silicone phase.Sensitivity to amine induced drift scales inversely with sodiumbicarbonate concentration over broad ranges of indicator concentrationand phosphate buffer concentration.

The optimized sensor sensitivity can also be obtained at a low sodiumbicarbonate concentration by using an indicator with a lower pKa. Thisoffers the advantage that for sensors containing only acid impurities,the amount of CO₂ conditioning drift can be reduced. In this case, theacid induced CO₂ conditioning drift is minimized by working at buffercompositions prepared from low concentrations of sodium bicarbonate andhigher concentrations of either the indicator, the phosphate buffer orboth. The high concentration of indicator and/or phosphate buffer servesto buffer the aqueous phase against large pH changes when going from anair equilibrated state to 6% CO₂. In this case, an optimized sensitivitycan still be achieved through proper choice of the carbonate, phosphateand indicator concentrations.

When there is a mixture of both acid and amine impurities in limitedamounts, drift can be reduced by choosing an intermediate concentrationof sodium bicarbonate to limit amine induced drift and a sufficientlyhigh concentration of indicator and/or phosphate buffer to limit acidinduced drift. In this case, an optimized sensitivity can still beachieved, but the change in pH on going from air to 6 volume percent CO₂is greatly reduced. Again, by increasing the concentration of thebuffers and increasing the pKa of the indicator, optimum sensitivity canbe maintained and amine induced drift can be reduced.

In choosing a dye for measuring carbon dioxide in blood, considerationis given to matching the pKa of the dye to the pH range of the aqueousphase induced by physical CO₂ levels. In constructing a gas sensor ofthis invention for use in sensing carbon dioxide gas in blood, we havefound that hydroxypyrene 3,6,8-trisulfonic acid has characteristicswhich are superior to beta-methylum-belliferone, althoughbeta-methylumbelliferone can also be used. Hydroxy-pyrene3,6,8-trisulfonic acid, hereinafter referred to as HPTS, which is aknown fluorescence dye for carbon dioxide, has a larger "Stokes shift"than does the umbelliferone compound. For use in fluorescencespectroscopy, this separates the excitation light from the emissionlight which improves the measurement of the emission light for improvedgas sensor performance. The hydroxypyrene trisulfonic acid can be usedas a free acid or as one of its salts as for instance an alkali oralkali earth salt.

Suitable indicator components for use in the present invention include:9-amino-6-chloro-2-methoxyacridine; 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein; 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein, acetoxymethyl ester;2',7'-bis-(2-carboxyethyl)-5-(and -6)-carboxyfluorescein, acetoxymethylester; 5-(and -6)-carboxy-2',7'-dichlorofluorescein; 5-(and-6)-carboxy-2',7'-dichlorofluorescein diacetate; 5-(and-6)-carboxy-4',5'-dimethylfluorescein; 5-(and-6)-carboxy-4',5'-dimethylfluorescein diacetate; 5-carboxyfluorescein;6-carboxyfluorescein; 5-(and -6)-carboxyfluorescein;5-carboxyfluorescein diacetate; 6-carboxyfluorescein diacetate;5-carboxyfluorescein diacetate, acetoxymethyl ester; 5-(and-6)-carboxyfluorescein diacetate; 5-(and -6)-carboxynaphthofluorescein;5-(and -6)-carboxynaphthofluorescein diacetate; 5-(and-6)-carboxySNAFL®-1, succinimidylester {5'(and 6')-succinimidylester-3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one};5-(and-6)-carboxySNAFL®-2, succinimidyl ester {5'(and6')-succinimidylester-9-chloro-3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one};carboxySNAFL®-1{5'(and6')-carboxy-3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one}; carboxySNAFL®-1 diacetate {5'(and6')-carboxy-3,10-diacetoxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one};carboxySNAFL®-2{5'(and6')-carboxy-9-chloro-3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one};carboxySNAFL®-2 diacetate {5'(and6')-carboxy-9-chloro-3,10-diacetoxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one};carboxySNAFL®-1 {5'(and6')-carboxy-10-dimethylamino-3-hydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one};carboxySNAFL®-1, AM acetate{3-acetoxy-5'-acetoxymethoxycarbonyl-10-dimethylamino-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one};carboxySNARF®-2 {5'(and6')-carboxy-10-diethylamino-3-hydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one}; carboxySNARF®-2, AM acetate{3-acetoxy-5'-acetoxymethoxycarbonyl-10-diethylamine-3-hydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one};carboxySNARF®-6 {5'(and6')-carboxy-10-diethylamino-3-hydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one};carboxySNARF®-X {5'(and6')-carboxy-3-hydroxytetrahydroquinolizino[1,9-hi]spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one};5-chloromethylfluorescein diacetate; 4-chloromethyl-7-hydroxycoumarin;Cl-NERF{4-[2-chloro-6-(ethylamino)-7-methyl-3-oxo-3H-xanthen-9-yl]-1,3-benzenedicarboxylic acid}; dextran, BCECF, 10,000 MW, anionic {dextran,2',7'-bis(2-carboxyethyl)-5(and 6)-carboxy-fluorescein, anionic};dextran, BCECF, 40,000 MW, anionic; dextran, BCECF, 70,000 MW, anionic;dextran, Cl-NERF, 10,000 MW, anionic; dextran, Cl-NERF, 70,000 MW,anionic; dextran, Cl-NERF, 10,000 MW, anionic, lysine fixable; dextran,DM-NERF, 10,000 MW, anionic {dextran,4-[2,7-dimethyl-6-(ethylamino)-3-oxo-3H-xanthen-9-yl]-1,3-benzenedicarboxylic acid, anionic}; dextran, DM-NERF, 70,000 MW, anionic;dextran, DM-NERF, 10,000 MW, anionic, lysine fixable; dextran,7-hydroxycoumarin, 10,000 MW, neutral; dextran, 7-hydroxycoumarin,70,000 MW, neutral; dextran, β-methylumbelliferone, 10,000 MW, neutral;dextran, β-methylumbelliferone, 70,000 MW, neutral; dextran, SNAFL®-2,10,000 MW, anionic {dextran,9-chloro-3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]3'-one,anionic}; dextran, SNAFL®-2, 70,000 MW, anionic {dextran,10-dimethylamino-3-hydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one,anionic}; dextran, SNARF®-1, 10,000 MW, anionic; dextran, SNARF®-1,70,000 MW, anionic; 1,4-dihydroxyphthalonitrile; DM-NERF{4-[2,7-dimethyl-6-ethylamino)-3-oxo-3H-xanthen-9-yl]1,3-benzenedicarboxylic acid}; fluorescein diacetate;8-hydroxypyrene-1,3,6-trisulfonic acid, trisodium salt;naphthofluorescein; naphthofluorescein diacetate; SNAFL®-1{3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one}; and SNAFL®-1, diacetate{3,10-diacetoxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one}.

Many of the above indicator compounds are commercially available fromMolecular Probes, Inc. "SNARF" and "SNAFL" are registered trademarks ofMolecular Probes, Inc. The structures of many of the aforementionedindicator compounds are listed in "Handbook of Fluorescent Probes andResearch Chemicals", 5th Edition, pages 129 to 141 (1992) by Richard P.Haugland. Also absorption dyes such as chlorophenol red, bromo cresolpurple, nitrophenol, bromo thymol blue, pinachorome and phenol red couldbe used.

Preferably, the concentration of the dye in the aqueous phase would befrom about 1 millimolar to about 20 millimolar with about a 2 to 8millimolar solution being typically used. Generally the concentration ofthe phosphate buffer in the aqueous phase would be from about 1millimolar to about 50 millimolar with about a 10 millimolar solutiontypically being used. Generally the concentration of the bicarbonatebuffer in the aqueous phase would be from about 5 millimolar to about200 millimolar, with about a 20 millimolar formal concentration beingused.

Other particular gasses of interest for the gas sensor of this inventioninclude ammonia, SO₂, and NO₂. For sensing ammonia a pH sensitive dyewould be solubilized in the aqueous phase. Gas exchange through thepolymeric phase and into the aqueous phase solubilize the ammonia gas inthe aqueous phase which interacts with the buffer ions. The dye chosenis one which is responsive to the concentrations of the ionic species ofthe ammonia in the aqueous phase, i.e., an acid-base responsive dye.Preferred for use in sensing ammonia is a ammonium chloride ion basedbuffer in the aqueous phase. Such a buffer can be chosen so as to have abuffer range compatible with the response range of the dye. Suitableindicators for use in measuring ammonium concentration include acridineorange, 1-hydroxypyrene-3,6,8-trisulphonate, and1-naphthol-4-sulphonate.

Certain properties of the emulsion or suspension between the aqueousphase and the polymeric precursor can be enhanced by adding additionalagents herein identified by the terminology "emulsification enhancementagents". These emulsification enhancement agents enhance certainmanufacturing properties such as shelf life of the gas sensorintermediates by stabilizing the emulsion and retarding dehydration ofthe aqueous phase. In general, these emulsification enhancement agentsare hydrophilic macro molecules. By retarding the dehydration of theaqueous phase and/or retarding break down of the emulsion or suspensionof the aqueous phase and the polymeric precursor, it is not mandatory toimmediately polymerize the aqueous phase-polymeric precursor emulsion orsuspension into the emulsoid gas sensor of the invention. With theaddition of the emulsification enhancement agents, the emulsion orsuspension of the aqueous phase and polymeric precursor is stable andcan be set aside for formation into the emulsoid gas sensor of theinvention at a later time. This reduces the need to adhere to a tightmanufacturing schedule and reduces or prevents the generation ofmanufacturing "scrap materials" which are economically wasteful. Ifused, the emulsification enhancement agent would be present at fromabout 5% to about 50% by weight per weight of the water of the aqueousphase. Typically about a 10-20% by weight of the emulsificationenhancement agent is used. The emulsification enhancement agents may,for example, thicken or increase the viscosity of the aqueous phase ormay alternatively create a thermodynamically stable emulsion by actingas a surfactant.

A material commonly used to thicken the aqueous phase,hydroxyethylcellulose ("HEC"), has been discovered to introduce asignificant contributor to drift (i.e., a source of migrating species).This is believed to be a consequence of residual sodium acetate speciespresent in the HEC thickener, as received from the manufacturer. We havediscovered that the use of alternative thickeners which aresubstantially free of migratable acid or base impurities substantiallyeliminates this drift contribution. Suitable thickeners for use in thepresent invention are those thickeners which when present in asufficient amount to provide the necessary or desired thickening to theaqueous phase are substantially free of partitioning species which canmigrate from the aqueous phase to the hydrophobic second phase and whichsubstantially affect the concentration dependent signal (i.e.,contribute to CO₂ conditioning drift). For example, we have discoveredthat poly(ethylene oxide) at 300,000 molecular weight (available fromUnion Carbide under the trademark Polyox WSR-N750) provides excellentresults and doesn't appreciably contribute to drift. Although the Polyoxmaterial is stated by the manufacturer to contain some residual ammonia,we found that this residual ammonia appears to have little or no impacton drift. It is believed that any residual ammonia in the Polyoxmaterial is either below the concentration necessary to cause drift, ornot retained by the sensor after manufacture of the emulsoid.

Alternatively, we have discovered that purification of theaforementioned HEC thickener (e.g., by a dialysis process to remove thesodium acetate) will eliminate the contribution by the thickener todrift in sensors that use HEC to thicken the aqueous phase.

Other suitable thickeners or emulsifiers (referred to collectively as"emulsification enhancement agents") can be utilized, as long as thepreviously stated criteria are met. For example, the use of a watersoluble polymer is envisioned. Suitable water soluble polymers include:water soluble polyethers such as the aforementioned poly(ethyleneoxide), and poly(ethylene glycol); water soluble vinyl or acryl polymersincluding, for example, poly(acrylamide), poly(dimethylacrylamide),poly(vinylalcohol), poly(vinylmethyl acetamide), poly(vinylpyrollidone),poly(styrene sulfonate), and poly(acrylamidomethylproponesulfonic acid),poly(hydroxyethylacrylate), poly(hydroxymethylacrylate), poly(acrylicacid), and poly(vinyl methyl ether); water soluble polysaccharidepolymers including, for example, dextran, xanthan gum, guar gum, alginicacid, and carageenen; water soluble cellulosic derivatives such as theaforementioned hydroxyethylcellulose, hydroxypropyl cellulose,carboxymethylcellulose, methylcellulose, hydroxypropyl(methylcellulose),and hydroxybutyl(methylcellulose); and other polymers such aspoly(ethyloxazoline) and poly(ethylene-imine). Copolymers of any of theabove or other polymers are also of possible use.

The suitability of any particular material should be tested relative tothe amount and processing conditions desired to be used. Some materials,such as the aforementioned poly(ethylene oxide), are suitable for usewithout further purification and are stable to a wide range ofprocessing conditions. In contrast, cellulosic derivatives (asexemplified by hydroxyethylcellulose) may need purification to reduceany contaminants to acceptable levels. The cellulosic materials arebelieved to be stable to a variety of processing conditions. In furthercontrast, polysaccharides (as exemplified by dextran) may be suitablefor use as received and without purification, but when exposed totypical conditions found in steam autoclave (>100 C.), which is a commonmethod of sterilization, are believed to be susceptible to degradationreleasing drift-inducing carboxylic acids. Thus, suitable materialsshould be evaluated for their purity (or the ease by which they may bepurified) and/or their ability to withstand autoclave conditions. Insome cases it may be advantageous to choose the higher molecular weightversion of a given water soluble polymer, in that this generallyprovides the highest viscosification for the lowest amount of polymer,thus allowing the amount of polymer added (with therein-containingimpurity) to be held to a minimum.

Notably, from the point of view of drift it is not necessarily requiredthat the water soluble polymer itself be strictly neutral, that is, freeof charges or ionizable groups, since any groups bonded to a watersoluble polymer may be substantially incapable of partitioning into thehydrophobic matrix. However, from the point of view of sensorfunctioning, the contribution of any ionizable polymer substituent tothe aqueous phase buffering capacity, or the contribution of such anionizable substituent, or any associated counterion, to the ionicstrength of the aqueous phase, may need to be taken into account.

It should be recognizable that emulsification enhancement agents such assurfactants may also be employed, subject to the above criteria. Thesemight include, for example, nonionic surfactants such as the Silwetfamily of polyethylene oxide/silicone surfactants sold by Union Carbide.Other surfactants are possible. Suitable surfactants include ionicsurfactants as long as any effect of the ions, either on the surfactantchain or mobile counterions, does not cause or promote drift.

A hydrophobic second phase (e.g., a polymeric phase) is chosen as acarrier for the aqueous phase and to maintain the individualmicro-compartments of the first phase in their dispersed form. Thesecond phase must be permeable to the gas of interest. It must also bepermeable to the wavelength or wavelengths of light utilized in themeasurement of the gas of interest. Further, since it is necessary tomaintain the aqueous phase isolated from the carrier fluid of the gas ofinterest, the second phase should be substantially impermeable to liquidwater. In order to isolate the indicator and/or any other ingredients inthe aqueous phase, the hydrophobic phase should also be impermeable toionic species. As for instance if the aqueous phase contains a buffer,it is important to maintain the ionic concentration of the buffer ionsconstant and not to dilute out or to increase the concentration of thesedesired buffer ions.

Because of their high gas permeability and ionized hydrogenimpermeability, silicone polymers are preferred for use as thehydrophobic secondary phase which separates the aqueous phase and themedium being analyzed. We have also discovered that certain hydrophobicsecondary phase materials used in traditional CO₂ sensors contribute todrift. It is believed that the traditional materials contain (or emitduring the curing process) partitioning species (such as acids or bases)which can migrate from the hydrophobic phase to the aqueous phase inresponse to a CO₂ induced pH change in the aqueous phase and therebycontribute to CO₂ conditioning drift. We have discovered that bycarefully selecting appropriate materials for use as the hydrophobicphase or by treating standard traditional materials to remove orimmobilize the partitioning species a drift free sensor can beconstructed.

In general, the polymeric phase can be prepared via severalpolymerization reactions. Aside from the above described addition typepolymerization, the polymeric phase can also be prepared via freeradical polymerization reactions using silicones having ethylenicallyunsaturated groups, via condensation polymerization reactions usingsilanol terminated silicones cross-linked with alkoxyl silanes usingcatalysts such as tin derivatives, or via photoinitiated polymerizationusing ultraviolet or visible light catalysts. Notably, certaincondensation cure silicones liberate acetic acid during the curingprocess. These materials are preferably avoided as the liberated aceticacid is believed to be an effective partitioning compound. Purificationof such materials after curing may eliminate these partitioning speciesand enable this type of silicone to be utilized.

In one presently preferred embodiment the polymeric phase is preparedvia a photoinitiated polymerization reaction and optionally followedwith a thermal polymerization reaction. This may be done using eitherUV, near IR, or visible light. In one embodiment a free-radicalinitiator is utilized to crosslink an acrylate or methacrylatefunctional silicone polymer. Alternatively, and preferably, a radiationactivated hydrosilation reaction is employed. For example, one mayemploy: (η⁴ -cyclooctadiene)diarylplatinum complexes (U.S. Pat. No.4,530,879, Drahnak, which is herein incorporated by reference); (η⁵-cyclopentadienyl)trialkylplatinum complexes (U.S. Pat. No. 4,510,094,Drahnak, which is herein incorporated by reference); or (η⁵-cyclopentadienyl)tri(σ-aliphatic)-platinum complexes and a sensitizerthat is capable of absorbing visible light (U.S. Pat. No. 4,916,169,Boardman et al., which is herein incorporated by reference) withtraditional siloxane polymers and crosslinkers.

The use of a photo-activated catalyst results in a decrease in scrap andwaste compared to typical silicone catalyst formulations which oftenprovide incompletely cured sensors. This is because traditional sensorsoften comprise silicone polymers which are cross-linked using a Karstedtcatalyst. This type of catalyst is often of variable quality and isprone to becoming poisoned. As a result much effort is expended toprepare sensors which then fail to adequately cure. Photo-activatedsystems are also preferred for their greater flexibility inmanufacturing. Traditional silicone systems require careful attention toworking time and setting time constraints. Great care must be taken tofully form the sensor within the working time of the silicone material.Failure to finish forming the sensor within the allotted time results inscrap product. Photo-activated materials are more convenient, since theactivation step can be delayed until the sensor is completely and fullyformed. This virtually eliminates waste due to premature setting.

Suitable photoinitiators for use in the present invention should notundesirably contribute to CO₂ conditioning drift or undesirablyinterfere with the transmission of either the excitation light signal orthe emission light signal through the sensor. Notably, we have not todate observed any measurable contribution to drift by either a typicalplatinum hydrosilylation catalyst or the aforementioned UV-activatedversions of this catalyst.

For use in forming a carbon dioxide gas sensor, polydimethysiloxane isparticularly preferred. We have discovered that a particularly usefulcommercial polydimethysiloxane provides sensors with substantially driftfree response. It is used in conjunction with a cross-linking agent anda platinum catalyst such as a Karstedt catalyst.

Various typical hydrosilylation-cure polydimethylsiloxanes werediscovered to contribute to drift. In contrast, one particular class ofsilicones (e.g., SYL OFF 7690 and 7691 from Dow Corning) exhibitedsubstantially lower drift. This preferred class is believed to includematerials made without an amine catalyst (in contrast to the traditionalamine catalysis route which is used for most commercially availablesilicones including those silicones listed in Table 1a as: "C", "D","E", and "F"). It is presently believed that the catalyst in SYL OFF7690 and 7691 may contribute less to the drift than the amine speciespresent in the other materials because it is either present at a lowerconcentration or because it is less able to reversibly partition inresponse to pH changes. Additionally, we found that the contribution bythe silicone materials to drift can be somewhat reduced even for theusual amine-catalyzed base polymers by heating them at elevatedtemperature and high vacuum (150° C., 2 mm Hg) for a period of time.Other purification/extraction techniques might also be used. Likewise,acid catalyzed materials, including many crosslinker materials, can bemade less prone to contribute to drift if heat treated or "stripped"prior to use in the sensor.

From about 1 gram to about 4 grams of the aqueous solution would beadded to about 10 grams of the polymeric precursor. Typically about 2grams of the aqueous phase per 10 grams of the polymeric precursor isused. The cross-linking agent would be added from about 2% to about 20%by weight of the polymeric precursor with approximately 5% by weightwith respect to the weight of the polymeric precursor typically beingused.

In addition to the previously mentioned emulsification enhancementagents, dispersing agents, such as fillers, can be added to thehydrophobic polymer phase if desired. Such agents serve to stabilize theinitially formed dispersion prior to final crosslinking or cure. Thesedispersing agents, when added to the hydrophobic phase, may also serveto enhance the structural characteristics of the hydrophobic phase aftercrosslinking. That is to say, the filler may serve to improve themechanical strength or integrity of the cured matrix. Suitablehydrophobic dispersing agents include fumed silica, precipitated silicaor finely divided silica. Depending on the polymeric phase material,catalyst molecules or particles might also remain in the polymeric phaseafter completion of the polymerization, as for instance metalliccatalyst particles.

The filler should also be selected such that the undesirablepartitioning species are not inadvertently brought into the sensorcomposition. For example, the Tullanox 500 fumed silica used in therepresentative formulation is known to contain a significant level of abasic species (introduced with a hydrophobic surface treatment).However, we have found this species to induce only a small, possiblynegligible, amount of drift. The residual base (believed to be ammonia)likely leaves the sensor during storage or during processing of thesensor. Nevertheless, as an extra precaution, one may preferably usefiller which has been heated under vacuum (e.g., for 12 hours at 150° C.and at 2 mm Hg) or a deammoniated filler such as Cabot TS530. Alsopreferred are silica fillers which have been hydrophobically treated byprocesses which do not result in the presence of basic impurities (suchas Cabot TS 610 or Cabot TS 720).

In an illustrative embodiment of the invention the dye is present in aquantity of about 0.01 grams per 2 mls. of said buffer solution, the dyein the buffer solution is added to the polymeric precursor in a quantityof about 2 mls. of the dye in the buffer solution to about 10 grams ofthe polymeric precursor. The cross-linking agent is added in a quantityof about 0.5 gram of the cross-linking agent per 10 grams of thepolymeric precursor and the catalyst is present in trace amounts (200ppm).

When the mixture of the aqueous phase and the polymeric precursor isemulsified, a suitable homogenizer such as a Virtis 23 homogenizer isused. The emulsification enhancement agent contributes to stability ofthe emulsion or suspension such that it has an increased shelf life.When it is desired to form the gas sensor of the invention, thecross-linker and/or catalyst is added (if they are not already presentin the polymeric precursor) or the sensor is exposed to visible light orUV light if a photosensitive initiator is present. These are gentlystirred into the emulsion and the resulting mixture then shaped andcured. A very simple gas sensor can be formed by simply depositing adrop of the mixture of the emulsion and the cross-linking agent onto theend of a fiber optic fiber and allowing it to cure into an emulsoiddirectly on the end of the fiber. Alternatively, the emulsion mixture ora sheet of sensor material formed from the emulsion as described incopending U.S. Patent application "Sensing Elements and Methods forMaking Same", Ser. No. 08/159,799 can be placed in a sensor holder or"cassette" to form a sensor.

Following emulsification, the aqueous phase is present in the polymericprecursor in micro-compartments which are generally smaller than 5microns. Typically a production gas sensor of the invention will havemicro compartments of the aqueous phase in the polymeric phase whereinthe majority of the population of the compartments will be on the orderof 2 microns. It is, of course, realized that the particles willactually be in a statistical range of particle sizes, some slightlylarger than the above noted sizes, some slightly smaller, depending onthe emulsification procedure and apparatus.

Seen in FIG. 1 is a drop 10 of the emulsion or suspension of the aqueousphase in the polymeric precursor. As is evident, the micro-compartments12 are dispersed in a uniform manner through the drop 10 of theemulsion.

For formation of a very simple gas sensor 14 of this invention, in FIG.2a, a drop of the above mixture is placed on the distal end 15 of anoptical fiber 16. The mixture of the cross-linking agent and thepolymeric precursor having the aqueous phase as an emulsion thereincures into an emulsoid 18 of the micro-compartments 20 of the aqueousphase in the polymeric material or carrier body 22. If desired, theemulsoid 18 can be retained on the end of the fiber 16 using a suitablesleeve 24. The sleeve 24 can be constructed from a suitable materialsuch as Teflon or the like. Further, to avoid light intensity changescaused by factors other than the changes in partial pressure of the gassensed, an overcoat 26 can be added as a layer over the exposedpositions of the emulsoid 18. For use with a fluorescent dye, theovercoat 26 is chosen to be opaque to the excitation light wavelengthλ_(ex) and to the emission light wavelength λ_(em) both of which aretransmitted along the same single optical fiber 16. A suitable materialfor the overcoat 26 would be vinyl end-capped poly(dimethyl) siloxaneimpregnated with carbon black.

As is evident in FIG. 2a, the size of the gas sensor 14 is dictated onlyby the optical fiber size. The gas sensor 14 thus formed is of asufficiently small size so as to be introducible directed into thecardiovascular system of a patient for direct real time measurement ofthe partial pressure of a blood gas such as carbon dioxide. If the fiberoptic fiber 16 of FIG. 2a is typically about 125 micron in diameter, itis evident that the emulsoid 18 is approximately equal to or less thanthis size in each of its orthogonally oriented width, height and depthdimensions. Other constructions of gas sensors are also possibleutilizing the emulsoid of this invention. It, of course, being realizedthat smaller sensors could be constructed by utilizing a smallerdiameter fiber optic cable.

By using the above noted gas sensor construction in conjunction withHPTS as a pH sensitive dye, determination time of the carbon dioxidepartial pressure is made in a time period of approximately one minute.This gas sensor can be autoclaved to sterilize it without detractingfrom or degrading its performance and during its use it is essentiallytemperature stable.

FIG. 2b shows a sensor 110 according to the present invention. Sensor110 is adapted to determine the concentration or partial pressure ofcarbon dioxide in blood. An optical fiber 112 is connected to anappropriate light transmitting apparatus 114, which is capable oftransmitting light at 410 and 460 nanometers. The light transmittingapparatus 114 generates the excitation light at these wavelengths. Theoptical fiber 112 is also connected to a light receiving apparatus 116,which, in turn, is connected to a conventional electronic processor 117.Located on the optical surface 118 of the optical fiber 112 is a matrix120 which is a carbon dioxide permeable material, such as a cross-linkedaddition cured siloxane polymer. Within the matrix 120 is a plurality ofmicro-compartments 121 comprising an aqueous phase including HPTSindicator dye. The highly carbon dioxide permeable matrix 120 adheres tothe optical surface 118 and slightly down along the sides 122 of the endof fiber 112. An opaque overcoating 124, comprising iron oxide pigmentdispersed in an addition cured polysiloxane, can then be applied overthe totality of the matrix 120 and down further along the side 122 ofthe fiber 112.

In use, sensor 110 functions as follows. The tip of optical fiber 112including matrix 120 and overcoating 124 is exposed or immersed inblood, the carbon dioxide concentration of which is to be determined.Light transmitting apparatus 114 transmits light at 410 nanometers tothe optical fiber 112. The excitation light at 410 nanometers causes thematrix 120 to fluoresce at 510 nm. In this case, the 410 nm light isabsorbed primarily by the acidic form of HPTS. Excited statedeprotonation follows, giving rise to 510 nm emission from the basicform of the dye. This emission is proportional to the amount of HPTSinitially present in the acidic form. As the concentration of carbondioxide in the blood increases, the pH of the aqueous phase drops andthe intensity of 510 nm emission associated with 410 nm excitationincreases. Light transmitting apparatus 114 then transmits light at 460nm to the optical fiber. The excitation light at 460 nm also causes thematrix 120 to fluoresce at 510 nm. In this case, the 460 nm light isabsorbed by the basic form of HPTS, which emits directly at 510 nm. Thisemission is proportional to the amount of dye initially present in thebasic form. As the concentrate of carbon dioxide in the blood increases,the intensity of 510 nm emission associated with the 460 nm excitationdecreases. The fluorescent emitted signals are transmitted from matrix120 through optical fiber 112 to light receiving apparatus 116.Processor 117 uses information received by light receiving apparatus 116on the longer emitted signal to determine a value of the carbon dioxideconcentration in the blood. Receipt and analysis of this fluorescentlight by light receiving apparatus 116 and processor 117 is carried outin a manner similar to that described in the above-referenced Lubbers,et al. U.S. Pat. No. RE 31,897 and in Heitzmann U.S. Pat. No. 4,557,900each of which is incorporated in its entirety herein by reference.Processor 117 uses information received by light receiving apparatus 116of the fluorescent signals emitted at 510 nanometers to develop a ratioof the emitted fluorescent signal associated with 460 nm excitation tothe fluorescent signal associated with 410 nm excitation. Using thisratio together with the above-noted carbon dioxide concentration,processor 117 can determine a corrected concentration of carbon dioxidein the blood to be analyzed. This corrected carbon dioxide concentrationis found to be accurate even if the optical fiber 112 is bent at one ormore points along its length and/or if other light transmissiondifficulties are encountered.

The above-noted procedure may occur periodically or even substantiallycontinuously to give substantially continuous carbon dioxideconcentration results. Of course, the transmission of the excitation at460 nanometers can take place before transmission of the excitation at410 nanometers. Also, by proper selection of the optical indicators,e.g., fluorescent dyes, the concentration of other components ofinterest can be determined. In addition, media other than blood can beanalyzed.

The optical fiber 112 may be in the form of a probe or a catheterinsertable into a blood vessel of a patient to provide continuouson-line in vivo monitoring of oxygen concentration in the blood.Alternately, the present sensor can be embodied in a flow-throughhousing as shown, for example, in the above-referenced Heitzmann patent,to provide extra corporeal monitoring of carbon dioxide concentration inthe blood. In this case it is often sufficient to monitor fluorescenceassociated only with the excitation at 460 or 410 nm.

FIG. 2c shows a sensor 210 according to the present invention. Sensor210 is adapted to determine the concentration or partial pressure ofcarbon dioxide in blood. An optical fiber 212 is connected to anappropriate light transmitting apparatus 214, which is capable oftransmitting light at 543 nanometers. The light transmitting apparatus214 generates the excitation light at this wavelength. The optical fiber212 is also connected to a light receiving apparatus 216, which, inturn, is connected to a conventional electronic processor 217. Locatedon the optical surface 218 of the optical fiber 212 is a matrix 220which is an carbon dioxide permeable material, such as a cross-linkedaddition cured siloxane polymer. Within the matrix 220 is a plurality ofmicro-compartments 221 comprising an aqueous phase including a dye(e.g., SNARF-6). The highly carbon dioxide permeable matrix 220 adheresto the optical surface 218 and slightly down along the sides 222 of theend of fiber 212. An opaque overcoating 224, comprising iron oxidepigment dispersed in an addition cured polysiloxane, can then be appliedover the totality of the matrix 220 and down further along the side 222of the fiber 212.

In use, sensor 210 functions as follows. The tip of optical fiber 212including matrix 220 and overcoating 224 is exposed or immersed inblood, the carbon dioxide concentration of which is to be determined.Light transmitting apparatus 214 transmits light at 543 nanometers tothe optical fiber 212. The excitation light at 543 nanometers causes thematrix 220 to fluoresce at two separate wavelengths. The emission at theshorter wavelength is associated with the acidic form of the indicator.The emission at the longer wavelength is associated with the basic formof the indicator. As the concentration of carbon dioxide in the bloodincreases, the pH of the aqueous compartment drops and the intensity ofthe short wavelength emission increases while the intensity of the longwavelength emission drops. Typically, the short wavelength emission ismeasured at 580 nm and the longer wavelength emission is measured at 630nm. Both the emissions at 580 nanometers and 630 nanometers aredependent on the concentration of carbon dioxide in the blood. Thefluorescent emitted signals are transmitted from matrix 220 throughoptical fiber 212 to light receiving apparatus 216. Processor 217 usesinformation received by light receiving apparatus 216 on the shorteremitted signal to determine a value of the carbon dioxide concentrationin the blood. Receipt and analysis of this fluorescent light by lightreceiving apparatus 216 and processor 217 is carried out in a mannersimilar to that described in the above-referenced Lubbers, et al. U.S.Pat. No. RE 31,897 and in Heitzmann U.S. Pat. No. 4,557,900 each ofwhich is incorporated in its entirety herein by reference. Processor 217uses information received by light receiving apparatus 216 of thefluorescent signal emitted at 580 nanometers to develop a ratio of theemitted fluorescent signal at 580 nanometers to the fluorescent signalat 630 nanometers. Using this ratio together with the above-noted carbondioxide concentration, processor 217 can determine a correctedconcentration of carbon dioxide in the blood to be analyzed. Thiscorrected carbon dioxide concentration is found to be accurate even ifthe optical fiber 212 is bent at one or more points along its lengthand/or if other light transmission difficulties are encountered.

The above-noted procedure may occur periodically or even substantiallycontinuously to give substantially continuous carbon dioxideconcentration results. Of course, the detection of the emission at 580nanometers can take place before detection of the emission at 630nanometers. Also, by proper selection of the optical indicators, e.g.,fluorescent dyes, the concentration of other components of interest canbe determined. In addition, media other than blood can be analyzed.

The optical fiber 212 may be in the form of a probe or a catheterinsertable into a blood vessel of a patient to provide continuouson-line in vivo monitoring of oxygen concentration in the blood.Alternately, the present sensor can be embodied in a flow-throughhousing as shown, for example, in the above-referenced Heitzmann patent,to provide extra corporeal monitoring of carbon dioxide concentration inthe blood.

An alternate embodiment, which is described with reference to FIG. 2d,involves a sensor apparatus making use of intensity modulated (sinewave) signals in the MHz range. In this embodiment, sensor 310 isadapted to determine the concentration or partial pressure of carbondioxide in blood. An optical fiber 312 is connected to an appropriatelight transmitting apparatus 314, which is capable of transmittingintensity modulated (sine wave) light in the MHz range. The lighttransmitting apparatus 314 generates the modulated excitation light atthis frequency. The optical fiber 312 is also connected to a lightreceiving apparatus 316, which, in turn, is connected to a conventionalelectronic processor 317. The light transmitting apparatus 314 includesa frequency generator (one or more frequencies simultaneously) linked toan electrically controlled light emitting structure, such as a lightemitting diode, a frequency doubled light emitting diode, or acombination of elements such as a continuous wave laser or incandescentlight source coupled to an acoustooptic modulator or electroopticmodulator, and the like. The light receiving apparatus 316 includes ahighly sensitive light detector having a rapid response time. Suitabledetectors include photomultiplier tubes such as those sold under thetrademark R928 by Hamamatsu Photonics K.K., Hamamatsu, Japan, as well asavalanche photodiodes and microchannel plates, also available from thesame supplier. Using techniques well known in the art, heterodynedetection can be implemented by modulating the detector sensitivity at afrequency, equal to the fundamental modulation frequency, F_(f) in theMHz regime, plus or minus a heterodyne modulation frequency F_(h) in theHz or kHz region. The processor 317 may include, for example, an analogto digital converter coupled by a direct memory access device to acomputer, or an analog phase comparator circuit known to those skilledin the art, and the like. The SLM 48000MHF Fourier TransformSpectrofluorometer manufactured by SLM-Aminco in conjunction with a HeNelaser provides frequency modulated light generation, light receivingapparatus and processor capability to perform the methods outlinedherein; to measure phase shifts, demodulation factors, or both at eithera single modulation frequency or simultaneously at several modulationfrequencies. Commercial software is available to apply a well-knowndigital fast Fourier transform to the data and to interpret phase anddemodulation data at multiple modulation frequencies in terms of adistribution of emission lifetimes and relative contributions.

Located on the optical surface 318 of the optical fiber 312 is a matrix320 which is an carbon dioxide permeable material, such as across-linked addition cured siloxane polymer which is similar to thepolymer described previously and containing a plurality ofmicro-compartments 321 comprising an aqueous phase comprising, forexample, SNARF-6 dye (or any other suitable lifetime based pHindicator). The highly oxygen permeable matrix 320 adheres to theoptical surface 318 and slightly down along the sides 322 of the end offiber 312. An opaque overcoating 324, comprising iron oxide pigmentdispersed in an addition cured polysiloxane, can then be applied overthe totality of the matrix 320 and down further along the side 322 ofthe fiber 312.

In use, sensor 310 functions as follows. The tip of optical fiber 312including matrix 320 and overcoating 324 is exposed or immersed inblood, the carbon dioxide concentration of which is to be determined.Light transmitting apparatus 314 transmits light at 50 MHz and 543 nm tothe optical fiber 312. This excitation light causes the matrix 320 tofluoresce at 610 mm, an isobestic point for emission from the acid andbase forms of SNARF-6. The fluorescent emission is sine wave modulated.The emission lifetime for the acidic form of the dye is longer than theemission lifetime of the basic form of the dye. As the concentration ofcarbon dioxide in the blood increases, the pH of the aqueous compartmentdrops and the phase shift increases while the demodulation factordecreases.

The fluorescent emitted signal is transmitted from matrix 320 throughoptical fiber 312 to light receiving apparatus 316. Processor 317 usesinformation received by light receiving apparatus 316 on the emittedsignal to determine the extent of the phase shift and/or thedemodulation factor of this emitted signal. The extent of this phaseshift and/or this demodulation factor is dependent on the concentrationof carbon dioxide in the blood. Thus, by determining the extent of thisphase shift and/or this demodulation factor, values of the carbondioxide concentration in the blood can be obtained. Transmission,receipt and analysis of this modulated signal by light transmittingapparatus 314, light receiving apparatus 316 and processor 317 may becarried out using equipment and in a manner similar to that described inGratton U.S. Pat. No. 4,840,485 which is incorporated in its entiretyherein by reference.

The above-noted procedure may occur periodically or even substantiallycontinuously to give substantially continuous oxygen concentrationresults. 0f course, by proper selection of the optical indicators, e.g.,fluorescent dyes, the concentration of other components of interest canbe determined. In addition, media other than blood can be analyzed.

The optical fiber 312 may be in the form of a probe or a catheterinsertable into a blood vessel of a patient to provide continuouson-line in vivo monitoring of oxygen concentration in the blood.Alternately, the present sensor can be embodied in a flow-throughhousing as shown, for example, in the above-referenced Heitzmann patent,to provide extra corporeal monitoring of oxygen concentration in theblood. In addition, the light transmitting apparatus 314 and/or lightreceiving apparatus 316 may be embodied in the flow-through housingwithout an intermediate optical fiber.

Seen in FIG. 5 is a schematic representation of an alternative CO₂sensor of the present invention. In this embodiment the gas sensor 30comprises a "cassette" or sensor holder 32 having a well 34. The well 34is open at one end, includes a bottom end wall 36 and a side wall 38. Adrop of emulsion 10 is placed in the well 34 and cured to form anemulsoid 40. An opaque layer 42 can be added as a layer over the exposedpositions of the emulsoid 40. In operation, a medium such as blood isbrought in contact with the exposed position of the emulsoid 40 (oralternatively in contact with the opaque layer 42). An excitation signalis transmitted through an optical fiber 44 which causes an emissionsignal from the indicator component. Alternatively, instead of using anoptical fiber 44 to transmit the excitation and emission signals, onemight either directly embed an LED and/or photodetector in the cassetteor place an LED and/or photodetector in contact with the cassette (notshown).

The following examples are offered to aid in the understanding of thepresent invention and are not to be construed as limiting the scopethereof. Unless otherwise indicated, all parts and percentages are byweight except that CO₂ gas compositions are expressed by volume percentor in terms of partial pressure in mm Hg.

EXAMPLES Example 1

An aqueous solution is prepared containing 6.36 mMhydroxypyrenetrisulfonate (trisodium salt), 6.6 mM sodium carbonatebuffer, 71 mM sodium chloride and 10% by weight of poly(ethylene oxide)having a viscosity average molecular weight of 300,000.

0.5 gm of this solution is added to 2.0 gm of a vinyl terminatedpoly(dimethylsiloxane) sold by Dow Corning under the trademark SYL OFF7690, to which is added 0.075 gm of fumed silica, sold by Cabot underthe trademark TS-530. This fumed silica is deammoniated prior to beingsold.

This mixture is homogenized three (3) times for 20 seconds each using aTissue-Tearor high shear homogenizer, with the sample being brieflywater cooled between each treatment. The resulting dispersion isdegassed for one (1) minute under vacuum. 200 ppm (by weight) of anultraviolet light activated hydrosilylation catalyst component(cyclopentadienyl trimethyl platinum) is then added and 0.025 gm of apoly (dimethyl) (methylhydro) siloxane, sold by Dow Corning under thetrademark SYL OFF-7678, is stirred in. This mixture has a silicon-bondedhydride/vinyl mole ratio of about 1.5. The mixture is again degassed.The degassed mixture is then cast into two (2) cavities of a sensorcassette made of polycarbonate. The cassette is then exposed to anultraviolet sun lamp for 1.5 minutes on each side (approx. 15 mJ/cm²) toactivate the catalyst component thereby causing the mixture to crosslinkor "cure". After this, the cassette is placed in an oven at 60° C. for20 minutes to finish the curing of the mixture. The sensor cassette isthen stored in the dark in normal saline until tested.

The amount of "CO₂ conditioning drift" a particular sensor exhibits ismeasured using the following standard protocol. First, the sensor isequilibrated in an air equilibrated medium at 37° C. using arecirculating aqueous loop sparged with air (i.e., having 0.25 mm CO₂)By "equilibrated" is meant that the sensor is exposed to the medium(e.g., using an air sparged aqueous loop) for a sufficient time periodsuch that it becomes fully adjusted to the medium (i.e., the sensor isin a thermodynamic "rest" state where it no longer is "adjusting" to anew state, e.g., such as when partitioning species react to a new stateand are in flux within a sensor). This air equilibrated rest state isdesigned to simulate the initial conditions a sensor exhibits when it isfirst manufactured or when the sensor is exposed to an air equilibratedmedium such as a saline solution drip line. The sensor is then attachedto a suitable excitation and detection assembly and a baselinemeasurement is obtained. The baseline measurement should be essentiallyconstant and may, for example, be expressed in units of intensity orvoltage which are then easily converted to pCO₂ (units of mm Hg) bycomparison to a calibration curve.

After the sensor has come into a condition of equilibration with the airequilibrated medium a step change is made to a new CO₂ level by exposingthe sensor to a medium which has been equilibrated with a gas having apCO₂ of 45.6 mm (i.e., a gas having 6 volume percent CO₂). This new CO₂level approximates a typical physiological level in a patient. This stepchange can be accomplished by quickly moving the sensor from the airequilibrated medium to a medium which is equilibrated with a gas havinga pCO₂ of 45.6 mm Hg. Alternatively, the medium surrounding the sensorcan be quickly changed.

Typically, after a rapid (less than about 5 minutes) initial signalresponse to the new CO₂ level, the sensor exhibits a slow CO₂conditioning drift back towards the baseline signal level. This drift ismost easily and reproducibly measured and reported as apparent "mm CO₂drift" per hour over the course of a three hour exposure to the 45.6 mmCO₂ equilibrated medium. That is, the measured mm CO₂ at the end of the"physiological" exposure period (i.e., 3 hours at pCO₂ =45.6 mm) minusthe initial measured mm CO₂ just after the initial rapid response isdivided by the number of hours of physiological exposure (i.e., 3). Thisis illustrated in FIG. 3b. The mm CO₂ drift per hour is equal to C/3.The percent mm drift per hour is equal to C/D·100/3. The drift may alsobe measured and reported as a percent of the change from the initialsignal value of the sensor per hour over the course of a three hourexposure to the 6 volume percent CO₂ medium (although for purposes ofthis invention the method described in FIG. 3b should be employed). Thatis, the signal reading at the end of the "physiological" exposure period(i.e., three hours at 6 vol. % CO₂) minus the "initial" signal readingjust after the initial rapid response is divided by the initial signalreading, multiplied by 100, and then divided by the number of hours ofphysiological exposure (i.e., 3). This is illustrated in FIG. 3a. Thepercent drift is equal to B/A·100/3. The units of drift are the sameunits as used for the signal itself.

Note that for most sensors the drift is not linear over the three hourperiod. Thus, the slope of the drift line varies. For purposes ofcomparison, the mm CO₂ drift will (unless otherwise noted) be calculatedas the average mm CO₂ drift per hour over the stated three hour period.

Also of importance for many diagnostic procedures is to minimize thetotal amount of CO₂ conditioning drift. As previously mentioned, thepartitioning species react to a step change in pH. For sensors which aremoved from a low pCO₂ medium to a higher pCO₂ medium this step changeinduces migration of various partitioning species. As a result of thismigration a new equilibrium is reached. Eventually, the migration stopsand the sensor is again stable (i.e., the signal has reached a plateau).The total mm CO₂ drift to reach this plateau is of particular importancefor sensors which must remain in service for prolonged periods. Onemethod of determining this plateau is to apply an exponential functionto the 3 hour drift data and extrapolate the data to longer time periods(e.g., 24 hours). From this analysis one can approximate the total mmCO₂ drift.

For sensing gases other than CO₂, one may measure the analyteconditioning drift in an analogous manner. The drift is calculated byfirst conditioning the sensor in an analyte free medium, moving thesensor to a test medium containing the analyte, and measuring the driftaway from the initial measured value. For comparison purposes, the testmedium should have a concentration of analyte at the midpoint of thesensor's operating range.

The sensing elements made in accordance with the above-describedprocedure (labeled as Run #1 in Table 1a) exhibit satisfactorysensitivity and time response, while typically exhibiting drift withinthe range of plus or minus 2.0% per hour (using the calculation methoddescribed in FIG. 3b). This level of drift is significantly lower thanthat of many typical representative formulations, e.g., Run 17, andapproaches the clinical and/or practical limitations of the driftmeasuring apparatus and technique.

Run 1 was repeated using different materials, untreated or treated indifferent ways, as described in Table 1a. The selection or treatment ofone or more of the components of the sensor is conducted to remove oneor more of certain weakly acidic components and/or weakly basiccomponents from the component or components. Each of the sensingelements produced was tested a number of times in accordance with thetest procedure previously described (note that Runs #6 and 16 used an 8volume percent CO₂ gas rather than a 6 volume percent CO₂ gas). Resultsof these tests are presented in Table 1a.

                                      TABLE 1a                                    __________________________________________________________________________    Silicone     Crosslinker                                                                           Silica      Drift.sup.7                                  Run                                                                              Type.sup.1                                                                        Treatment.sup.2                                                                     Type.sup.3                                                                        Ratio.sup.4                                                                       Treatment.sup.5                                                                     Viscosifier.sup.6                                                                   (%/hr)                                       __________________________________________________________________________     1 A   --    I   1.5:1                                                                             deamm.                                                                              PEO   <-2.0                                         2 A   150° C.                                                                      I   1.5:1                                                                             150° C.                                                                      PEO   -0.94                                         3 A   150° C.                                                                      I   1.5:1                                                                             150° C.                                                                      PEO   -0.52                                         4 A   150° C.                                                                      I   1.5:1                                                                             150° C.                                                                      PEO   0.81                                          5 A   --    I   1.5:1                                                                             150° C.                                                                      PEO   0.32                                          6 A   --    I   1.5:1                                                                             150° C.                                                                      PEO   -0.30                                         7 A   150° C.                                                                      I    10:1                                                                             150° C.                                                                      PEO   -2.87                                         8 A   --    II    2:1                                                                             deamm.                                                                              PEO   -1.97                                         9 A   --    II    2:1                                                                             deamm.                                                                              PEO   -0.78                                        10 B   --    I   1.5:1                                                                             deamm.                                                                              PEO   -3.61                                        11 C   150° C.                                                                      I     8:1                                                                             150° C.                                                                      PEO   -0.81                                        12 C   150° C.                                                                      III  10:1                                                                             150° C.                                                                      PEO   -3.20                                        13 C   150° C.                                                                      III  15:1                                                                             150° C.                                                                      PEO   -6.55                                        14 C   --    III  10:1                                                                             150° C.                                                                      PEO   -5.22                                        15 C   150° C.                                                                      III  10:1                                                                             --    PEO   -3.84                                        16 C   --    I     8:1                                                                             --    PEO   -3.36                                        17 C   --    III   8:1                                                                             150° C.                                                                      HEC   -16.5                                        18 D   --    IV   10:1                                                                             150° C.                                                                      HEC   -16.7                                        19 E   --    V    10:1                                                                             150° C.                                                                      HEC   -15.4                                        20 F   --    VI  --  --    HEC   -15.4                                        21 A   --    I   1.5:1                                                                             150° C.                                                                      HEC   -8.92                                        22 A   --    I   1.5:1                                                                             deamm.                                                                              .sup. HEC.sup.8                                                                     -1.40                                        23 G   --    III   8:1                                                                             deamm.                                                                              PEO   -3.30                                        __________________________________________________________________________     Footnotes for Table 1a:                                                       .sup.1 Silicone A is a vinyl functional poly(dimethylsiloxane) sold by Do     Corning under the trademark SYL OFF 7690.                                     Silicone B is a vinyl terminated poly(dimethylsiloxane) sold by Dow           Corning under the trademark SYL OFF 7691.                                     Silicone C is a vinyl terminated poly(dimethylsiloxane) sold by Petrarch      Systems under the trademark Petrarch PS 443.                                  Silicone D is a vinyl terminated poly(dimethylsiloxane) sold by Union         Carbide under the trademark Y7942.                                            Silicone E is a vinyl terminated poly(dimethylsiloxane) sold by NuSil         Silicone Technology under the trademark NuSil PLY7500.                        Silicone F is a vinyl terminated poly(dimethylsiloxane) sold by Dow           Corning under the trademark Q72218 part A.                                    Silicone G is a vinyl terminated poly(dimethylsiloxane) sold by Petrarch      under the trademark PS783.                                                    .sup.2 The heat treated samples were preprared using silicone polymer         materials which were heated to 150° C. at 2 mm Hg for 12 hours.        .sup.3 Crosslinker I is a poly(dimethyl) (methylhydro) siloxane sold by       Dow Corning under the trademark SYL OFF 7678.                                 Crosslinker II is a silyhydride compound prepared as described in Example     1 of U.S. Pat. No. 4,822,687.                                                 Crosslinker III is a poly(dimethyl) (methylhydro) siloxane sold by            Petrarch Systems under the trademark PS 123.                                  Crosslinker IV is a poly(dimethyl) (methylhydro) siloxane sold by Union       Carbide under the trademark Y 12247.                                          Crosslinker V is a poly(dimethyl) (methylhydro) siloxane sold by NuSil        Silicone Technology under the trademark NuSil XL123.                          Crosslinker VI is a poly(dimethyl) (methylhydro) siloxane sold by Dow         Corning under the trademark Q72218 part B.                                    .sup.4 The ratio following the crosslinker designation refers to the mole     ratio of siliconbound hydride groups to vinyl groups in the precursor         mixture. The crosslinker of Run #9 was further "stripped" by being fed at     10 ml/minute through a wipedfilm evaporator maintained at 230° C.      .sup.5 Each of the samples designated with a "--" or a "150° C."       contained a fumed silica sold by Tulco under the trademark Tullanox 500.      The heat treated samples were heated to 150° C. at 2 mm Hg for 12      hours. Each of the samples designated "deamm." contained a deammoniated       fumed silica sold by Cabot under the trademark TS530. This fumed silica i     stated by the manufacturer to be a more completely deammoniated version o     the Tullanox 500 fumed sili ca.                                               .sup.6 Each of the samples contained a "viscosifier" in the aqueous phase     By "PEO" is meant poly(ethylene oxide) having a molecular weight of           300,000 and available from Union Carbide under the trademark WSR N750. By     "HEC"  is meant hydroxyethylcellulose having a middleviscosity and            available from Fluka under the trademark "Hydroxyethylcellulose middle        viscosity 1". Analysis of the HEC as received indicates that it contains      sodium acetate and that a 10% s olution of HEC in water would contain 3.3     mM sodium acetate.                                                            .sup.7 The drift value is an average drift value for at least two samples     for each run and was calculated using the procedure described by FIG. 3b.     Several of the runs were replicated more than twice.                          .sup.8 For this run the HEC was purified using the following process: To      20 gm HEC was added 400 ml deionized water. The solution was then dialyze     over a ten day period using SpectraPore 1 dialysis tubing (having a           molecular weight cutoff of about 6,000 to 8,000 and a 1.46 cm diameter).      The dialyzed material was then filtered through a 10 to 20 micron (μm)     filter and freeze dried.                                                 

In general, sensors made with PEO viscosifier in the aqueous phaseexhibited significantly less drift compared to sensors made withuntreated HEC viscosifier. As previously mentioned, HEC is believed tocontain residual amounts of sodium acetate which can serve as apartitioning species. Purification of the HEC material lessens driftconsiderably as illustrated by comparison of Runs #21 and 22.

In general, Silicones A and B advantageously exhibit lower contributionto drift than do Silicones C, D, and E. Silicones A and B are believedto be manufactured via a different technique than the amine catalysisroute used in the manufacture of many commercially available silicones.Thus, the decreased contribution to drift might reflect a lower level ofpartitioning species (e.g., a silicone which is free of amines) as aresult of the different catalysis process. Alternatively, thesesilicones may contain partitioning species which are more easily removedduring sensor construction and fabrication than the traditional siliconematerials.

Somewhat better drift performance is obtained using Silicone A versusSilicone B. Since silicones A and B are believed to be made using thesame general method of manufacture, the differences in drift observedwhen using the two polymers may possibly reflect differences in somestructural characteristic of the polymer which affects the rate ofpartitioning of impurities into the cured polymer network.

In addition, the drift is somewhat reduced when using conventionalamine-catalyzed silicone based polymers by heat treating the silicone atelevated temperature and high vacuum, as evidenced by comparison of Runs12 and 14. Other purification techniques can also be employed to removeacidic and/or basic species from the components making up the sensingcomposition precursor.

As seen in Table 1a, it is evident that both the specific identity ofthe crosslinker and the ratio of hydride to vinyl can affect drift. Forexample, in general crosslinker I exhibits less contribution to driftthan crosslinker III. This may reflect a difference in residual catalyst(typically acid catalysts are used in the synthesis of suchcrosslinkers). In addition, comparison of Run 4 to Run 7, and Run 12 toRun 13, reveal that lower hydride to vinyl ratios provides sensors withless drift. Finally, comparison of Run 8 to Run 9 indicates that furtherpurification of a given crosslinker may serve to further reduce drift.

While the fumed silica used in a typical comparative formulation isknown to contain a significant level of residual base resulting from ahydrophobic surface treatment, the present example demonstrates thatthis residual base induces only a small, possibly negligible, amount ofdrift (e.g., compare Run 12 to Run 15). The residual base is believed tobe ammonia, which likely leaves the final sensor during storage.However, to lessen the possibility of contribution of this species tothe drift, the fumed silica can be heated under high vacuum or adeammoniated fumed silica can be used.

Referring to FIGS. 4a and 4b, a comparison of two runs of Example 1 areillustrated. FIG. 4a corresponds to one experimental measurement of thesensor of Run #20. The effect of CO₂ conditioning drift is evident andillustrates the problem encountered in traditional sensors. FIG. 4bcorresponds to one experimental measurement of the sensor of Run #5. Asillustrated by FIG. 4b, the sensor of Run #5 exhibits a stable signalwith negligible drift over time.

Example 2

An aqueous solution is formed by dissolving 11.8 mg ofhydroxypyrenetrisulfonate (trisodium salt), 375 mg of poly (ethyleneoxide) having a molecular weight of 300,000 (sold by Union Carbide underthe trademark WSR N-750), 31.4 mg of trisodium phosphate dodecahydrate,and 59.6 mg of sodium chloride in enough water to make 7.5 gm ofsolution.

A silicone mixture is prepared by combining 16.4 mg of cyclopentadienyltrimethyl platinum, (an ultraviolet light activated catalyst component),0.49 gm of fumed silica, sold by Cabot under the trademark TS-530, and15.91 gm of vinyl dimethyl end-capped poly(dimethyl siloxane) having aviscosity of 1000 centistokes sold by NuSil Silicone Technology underthe trademark PLY 7500.

The aqueous solution is combined with the silicone mixture, and 1.1 gmof poly(dimethyl) (methylhydro) siloxane having a molecular weight ofabout 2100 and a silyl hydride group content of about 30%, sold by NuSilSilicone Technology under the trademark XL 123 is added. Thiscombination is then processed with a Vertis Cyclone IQ homogenizer toform the sensing composition precursor.

A transparent polycarbonate web (available from Miles Inc. under thetrademark DE 1-1), having a thickness of about 0.0127 cm, is coated withan adhesion enhancement component (available from Dow Corning under thetrademark 1205 Prime Coat) derived from a mixture containing water,1.25% by weight of colloidal silica particles, 0.11% by weight ofaminopropyltriethoxysilane, 0.05% by weight ammonium hydroxide and 0.03%by weight of a surfactant sold by Rohm and Haas under the trademarkTriton X-1000.

Using a coating apparatus sold by Hirano under the trademark M-200, thesensing composition precursor is continuously coated on one side of theweb. In this coating operation (and all other coating operationsdescribed herein using this coating apparatus), the web is moved, at aconstant speed, in proximity to and under the coating apparatus which isstationary. This precursor coating has a substantially uniform thicknessof about 0.0025 cm. The sensing composition precursor is cured byexposing the coating to about 70 mJ cm⁻² of ultraviolet (365 nm) light.The coating is further cured by exposure at about 90° C. for about 2minutes to form the sensing composition.

An opaque film precursor is prepared by mixing 12 gm of a 12% by weightdispersion of carbon black, sold by Cabot under the trademark Regal 99R,in vinyl end-capped poly(dimethyl)siloxane, having a viscosity of 500centistokes (sold by NuSil Silicone Technology under the trademark PLY7501), with 26.6 mg of a platinum catalyst solution sold by NusilSilicone Technology under the trademark Cat50, 6.5 mg of apolymerization inhibitor sold by Nusil Silicone Technology under thetrademark XL119, and 0.6 gm of the poly(dimethyl) (methylhydro) siloxanenoted above. Using the coating apparatus noted above, this opaqueprecursor is continuously coated on the sensing layer in a substantiallyuniform coating with a thickness of about 0.0013 cm. The opaque filmprecursor is cured by exposure at 70° C. for 2 minutes to form theopaque film.

Individual carbon dioxide sensing elements are cut from the transparentweb/sensing composition/opaque film composite. Each sensing elementincludes a generally circularly shaped portion of the transparent web, agenerally circularly shaped portion of an opaque film and a thin,generally circularly shaped layer of the sensing compositiontherebetween. Such an individual sensing element is illustrated in FIGS.7 and 8.

Using conventional testing procedures, a small, representative sample ofthe individual carbon dioxide sensing elements is tested to determine ifsuch elements meet product specifications, that is if such elements areeffective for accurately and reliably sensing the concentration ofcarbon dioxide in human blood. These individual carbon dioxide sensingelements are found to be satisfactory based on this testing procedure.

Using a transparent silicone-based adhesive sold by Dow Corning underthe trademark 3140, one of these carbon dioxide sensing elements isbonded into a fight circular cylindrical open ended well, having adiameter of 0.3175 cm and a depth of 0.025±0.0025 cm, formed in apolycarbonate cassette so that the transparent web layer is facing thebottom of the well. Prior to bonding the sensing elements to thecassette, the walls of the well are contacted with a priming agent, soldby Dow Corning under the trademark 1205, to promote adhesion between thesensing element and the polycarbonate cassette.

The thus produced carbon dioxide sensor is effective in determining theconcentration of carbon dioxide in blood brought into contact with theopaque layer.

FIGS. 7 and 8 illustrate the use of a sensing element produced inExample 2 in determining carbon dioxide concentrations.

As shown in FIG. 7, this individual sensing element 50 is placed intowell 54 containing a transparent, silicone-based adhesive 57. Well 54 isopen at one end, includes a fight circular cylindrical side wall 55 anda circular bottom end wall 58. The size of well 54 is such that theindividual sensing element 50 and silicone-based adhesive layer 57completely fill the well. Individual sensing element 50 is placed inwell 54 so that the transparent web layer 64 faces the bottom end wall58 of well 54. The opaque layer 62 includes an exposed surface 63 whichis raised relative to the inner surface 70 of sensor holder 56. Theopaque layer 62 substantially shields sensing composition layer 66 fromdirect contact with the medium, e.g., blood, to be monitored. Dependingon the specific sensing application involved, the exposed surface of theopaque layer can be recessed relative to, or flush with, the innersurface of the sensor holder.

Referring now to FIG. 8, in use sensor holder 56, made of a transparentpolycarbonate material, is placed in abutting relation to optical fiber72. Optical fiber 72 provides excitation light of appropriate wavelengthfrom light transmitting apparatus 74 to excite the sensing component inthe sensing composition layer 66 to fluoresce and provide a signalcharacteristic of the concentration of carbon dioxide located in themedium in contact with the opaque film 62. This optical fiber 72 alsotransmits the signal which is emitted from the sensing component andpasses such signal to a light receiving apparatus 76, which processes oranalyzes this emitted signal, e.g., as described in Lubbers et al. U.S.Pat. No. RE. 31,879, Heitzmann U.S. Pat. No. 4,557,900, and/or copendingU.S. patent applications Ser. Nos. 08/136,967 and 08/137,289 todetermine the concentration of carbon dioxide in this medium.

Over a period of time, the individual sensing element 50 providesconsistent, e.g., substantially "drift" free, signals which are reliablycorrelated to the true and accurate concentration of carbon dioxide inthe blood in contact with the opaque layer 62.

Example 3

Referring now to FIG. 6, an aqueous dye solution 82 was prepared asdescribed in Example 1, Run #17. The dye solution was then sorbed intothe pores of an asymmetric nylon membrane 80 made according to themethod disclosed in Example 24 of U.S. Pat. No. 5,006,247, which isherein incorporated by reference. The dye solution/nylon membraneconstruction was then placed large pore side down into a sensor holder56 having a fight circular open ended well with a diameter of 0.4 cm anda depth of 0.013 cm. An overcoating of poly(dimethylsiloxane) 84 asdescribed in Example 1, Run #17 was then applied and cured atop theupper surface of the nylon membrane (i.e., the surface having thesmall-pore openings 86). When tested according to the proceduredescribed in Example 1 and FIG. 3b (but using an 8.0 volume percent CO₂gas instead of the 6.0 volume percent CO₂ gas) this sensor exhibitedless than -2% drift per hour, while maintaining excellent sensitivityand time response.

Nylon differs substantially from silicone in that small molecules (suchas gases, organic vapors, and organic liquids) are much less soluble andmuch less diffusible in nylon than in silicone. Therefore the ability ofspecies to partition from an aqueous phase into a nylon phase is greatlyreduced compared to partitioning into a silicone phase. However, sincethe permeability of CO₂ through nylon is small, sensors using nylon asthe hydrophobic phase or matrix would generally exhibit very slowresponse times. The asymmetric nylon membrane described herein possessesthe unique advantage of having large structured pores which pass throughthe membrane and out one surface, but which have extremely small poreopenings at the other surface. Thus, once the small pore opening surfaceis overcoated with silicone (achieving complete encapsulation of theaqueous phase), it is believed that the presence of the small nylonsurface pores allow sufficient permeation of CO₂ through the siliconephase and into the encapsulated aqueous phase, but minimizes theinterfacial area of contact between the encapsulated aqueous phase andthe overlying silicone. This physical arrangement substantially reducesthe rate of drift even in the presence of titratable impurities.

Example 4

A sensing composition was prepared as described in Example 1, Run #1 andformed into several individual sensing elements. Each individual sensingelement (having a diameter of 0.4 cm and a thickness of 0.013 cm) wascovered with a layer of an optical isolation barrier. The opticalisolation barrier serves the function of reducing the detection ofextraneous light signals (i.e., stray light from external sources). Theoptical barrier in one case consisted of a black teflon film availablefrom Chemical Fabrics Corp. under the tradename DF 1100. This filmcontains, on one side, an adhesive-receptive surface which is believedto contain residual acidic species. The optical barrier in another caseconsisted of an identical film which had been washed with a 10 mMaqueous bicarbonate solution for 25 minutes. The sensor comprising thewashed optical barrier exhibited drift of -5.5% per hour when tested asdescribed in Example 1 and FIG. 3b. The sensor comprising the unwashedoptical barrier film exhibited drift of about -16.3% per hour. A controlsensor without an optical barrier film exhibited drift of about -0.15%per hour.

Example 5

Three sensing compositions were prepared as described in Example 1 andformed into several individual sensing elements. To each individualsensing element was added an optical isolation pigment as describedbelow. A fourth sensing element without optical isolation pigment servedas a control.

In a first case 0.2 wt. percent carbon black (Cabot Regal 99R) was addedto the sensing element composition described in Example 1, Run #1. Thecomposition was then formed, cured and tested as described in Example 1and FIG. 3b. This sensor exhibited -2.2% drift per hour.

In a second case 2.0 wt. percent carbon black (Monarch 700) was added tothe sensing element composition described in Example 1, Run #10. Thecomposition was then formed, cured and tested as described in Example 1and FIG. 3b. This sensor exhibited +1.3% drift per hour.

In a third case 9 wt. percent of a hydrophobically treated TiO₂ wasadded to the sensing element composition described in Example 1, Run #1.The TiO₂ material was treated with A-174 silane using an acetic acidcatalyst. It is likely that some residual acetic acid species may remainafter this treatment. The composition was then formed, cured and testedas described in Example 1 and FIG. 3b. This sensor exhibited -11.6%drift per hour.

In a fourth case a sensing element without isolation pigment wasprepared as described in Example 1, Run #1. This sensor exhibited -1.2%drift per hour.

Example 6

A 25 gm solution of 100 mM sodium bicarbonate, 10 mM HPTS dye, wasprepared. To this solution was added 5.0 gm of polyvinylpyrolidone("PVP" having a 40,000 molecular weight and available from AldrichChemical Co.). The mixture was held at 37° C. for 14 hours to ensuresolubilization of the PVP. The PVP-dye solution was then stored frozenuntil time of use.

0.46 gm of the above solution was added to 1.98 gm PS 783 silicone(available from Petrarch). A dispersion was formed by homogenizing themixture using a Tissue-Tearor high shear homoginizer at the highest rpmsetting. The mixture was homogenized three times (20 seconds each time).

To 0.51 gm of the above dispersion was added 5 microliters ofphotoactivatable hydrosilation catalyst solution and 0.046 gm PS 123cross-linker (available from Petrarch). The catalyst and crosslinkerwere thoroughly stirred in for one minute with a glass capillary tube.

The above mixture was cast into two wells of a sensor holder. The sensorholder and mixture were then exposed to a UV sunlamp for 4 minutes (30mJ/cm²). This exposure caused the sensing elements to cure or crosslinkto a nontacky condition. The sensors were then stored in anair-equilibrated aqueous saline medium until used.

In use the sensors were tested as follows. Two of the above cassetteswere placed into a test loop and brought into an air-equilibrated stateat 37° C. A step change was then introduced by moving the sensor to a8.4 volume percent CO₂ medium (pCO₂ =63.8 mm Hg). An initial fastintensity drop (within 4 minutes) was observed, followed by a gradualfurther downward drift in intensity over the course of the three hourexposure to the 8.4 volume percent CO₂ medium. The drift for four runswas calculated to be +13.9% per hour (41.7 percent over the three hourperiod of testing) when measured as described in FIG. 3b.

Examination of this formulation reveals that the components of thesilicone phase, and the formulation procedure used, are typical of thosecomponents and procedures that are expected to contribute to CO₂conditioning drift species. In particular, an amine-containing (i.e.,amine catalyzed) silicone is employed in combination with a crosslinkerwhich is demonstrated to contribute to drift. Furthermore, thehydride:vinyl ratio is high (8:1), which is demonstrated to furtherincrease drift. The sensor of Example 1, Run #23 illustrates the use ofthe above silicone phase formulation in combination with a "clean"aqueous phase (i.e., a PEO-HPTS-carbonate solution which is demonstratedto not substantially contribute to drift). The resulting measured driftof Run #23 is due to the silicone phase materials.

Since the sensor of this example exhibits a positive mm drift, it isbelieved that the partitioning species of this sensor (perhaps due tothe sensor's relatively high internal buffer concentration) mayirreversibly migrate out of the sensor and into the medium (i.e., that"saline drift" predominates over "CO₂ conditioning drift").

As used herein, a sensor's "positive mm" drift or instability can bequantified by: (1) placing the sensor in a medium having a pCO₂ of 45.6mm Hg and obtaining a first measurement of CO₂ partial pressure; (2)storing the sensor in a large capacity (i.e., a reservoir havingsufficient capacity so as to act as a "sink" for any species migratingfrom the sensor) medium (having a pCO₂ of 0.25 mm Hg and being free ofany partitioning species) for a period of time (e.g., 24 hours); (3)placing the sensor in a medium having a pCO₂ of 45.6 mm Hg and obtaininga second measurement of CO₂ partial pressure; and (4) comparing thefirst and second measurements (i.e., calculating the difference betweenthe measurements).

Thus, a longer term drift, corresponding to the irreversible leaching ofpartitionable impurities out of the sensor chemistry, may besuperimposed on the shorter term drift which corresponds to migration ofpartitionable impurities between the internal aqueous phase and thehydrophobic phase of the sensor.

Example 7

An alternative approach to producing CO₂ sensors involves attaching a pHsensitive dye to a hydrophilic polymer (e.g., bonding a pH sensitive dyeto a functionalized hydrophilic polymer or copolymerizing a pH sensitivedye with a hydrophilic monomer). Frequently, acrylate or methacrylatemonomers and/or polymers are employed. We have discovered that theseacrylate based monomers and polymers typically contain residualquantities of acids (e.g., acrylic acid or methacrylic acid) inhibitors,and stabilizers. Unless measures are taken to remove these species theresulting sensor may be subject to drift.

The contribution to drift of the undesirable contaminants in a givensubstance can be evaluated by exposing a drift-free sensor to thecontributing substance. In this example a low-drift CO₂ sensor (a filmcomprising an emulsoid as per Example 1, Run #1 and having a 0.4 cmdiameter and a 0.013 cm thickness) was placed in a small containerfilled with an aqueous solution (1 ml capacity). On the inside surfaceof one side of the small container was placed 0.015 gm of the substancebeing investigated. The distance between the sample-spiked surface andthe sensor is approximately 2.5 mm. The sensor was stored for five daysand then tested as described in Example 1.

In a first run, no substance was coated on the inside surface of thecontainer. The sensor had drift of 1.85 percent per hour when tested asper Example 1 and FIG. 3b. In a second run,2(2-ethoxyethoxy)ethylacrylate (available from Sartomer Company underthe tradename 256) was coated on the container. The sensor exhibiteddrift of 14.1 percent per hour. In a third run,tetrahydrofurfuralmethylacrylate (available from Sartomer Company underthe tradename 203) was coated on the container. This substance hadapproximately 200 ppm acrylic acid and 30 ppm methacrylic acid. Thesensor exhibited drift of 9.36 percent per hour. In a fourth run,tetrahydrofurfuralmethylacrylate (available from Scientific PolymerProducts under the tradename M-130) was coated on the container. Thissubstance had approximately 1.5 ppm acrylic acid. The sensor exhibiteddrift of 6.3 percent per hour. In a fifth run,tetrahydrofurfuralmethylacrylate (available from Scientific PolymerProducts under the tradename M-130 and "purified" by Scientific PolymerProducts to remove acidic species) was coated on the container. Thesensor exhibited drift of 4.8 percent per hour.

These data illustrate that, if acrylate-based monomers are used in CO₂sensor formulations (such as in the polymerization of hydrophilicmonomers to form "hydrogels") precautions must be taken to remove driftcontributing species from the monomer, or from the resulting polymerproduct.

Example 8

This example describes a CO₂ sensor configured for "on demand"monitoring. An aqueous mixture was prepared comprising 6.36 mM Na₃ HPTS,6.6 mM Na₂ CO₃, 78 mM NaCl, and 10 wt % hydroxyethylcellulose (availablefrom Fluka). The mixture was held at 37° C. overnight to ensure completemixing. To 0.5 gm of this dye mixture was added 2.0 gm PS 443 siliconebase polymer (available from Petrarch) and 0.075 g of Tullinox 500 fumedsilica filler. This mixture was homogenized and degassed for 3 minutesat 2 Torr. To prepare individual sensors, 0.5 g of the resultingdispersion was mixed with ten microliters of a thermally activatedplatinum catalyst solution (a 1 wt % solution of a Karstedt catalyst intoluene) and 0.25 gm of PS123 crosslinker (available from Petrarch). Toprepare each sensor, a small amount of the resulting mixture wassandwiched between a black teflon membrane and a glass disk substrate(approximately 5 mm in diameter). The resulting sensors were placed in a60° C. oven to crosslink. The black teflon membrane serves as an opticalisolator and consisted of teflon impregnated with carbon black and asilicone adherent layer. The black teflon membranes were washed withsodium carbonate buffer before use. The completed sensors was thenincorporated into a polycarbonate flow-through cassette by glueing theback of the glass disk substrate to the polycarbonate with a RTVsilicone adhesive. Completed sensors were stored in a phosphatecalibration buffer for several days before testing.

The sensors cassettes were tested in a saline drip line to mimicclinical monitoring of blood gases. Air equilibrated saline flowedthrough the sensor cassette at a rate of 1 ml/min between samplemeasurements. On-demand monitoring was accomplished by drawing a sampleof CO₂ equilibrated phosphate buffer solution up the saline drip lineand over the sensor. In clinical practice, blood would be drawn over thesensor from an arterial catheter. After five minutes, the phosphatebuffer was flushed from the sensor cassette with fresh saline solution,and a slow air equilibrated saline drip was resumed.

The CO₂ sensor was excited at 460 nm, an excitation maximum for thedeprotonated form of the HPTS indicator dye, and emission intensity wasmeasured at 510 nm. During saline drip, the HPTS indicator ispredominatly in its basic form and the fluorescence intensity is high.At elevated CO₂ levels, the aqueous compartment pH drops andfluorescence intensity is reduced. The sensor was calibrated at two CO₂concentrations and at 37° C. using phosphate calibration buffers spargedwith either 2.8 or 8.4 volume % CO₂. A calibration relationship wasdetermined.

FIG. 9 shows a typical response of a calibrated sensor. During thedaytime, an "on demand" measurement of 5 volume % CO₂ sparged phosphatebuffer was made once every hour. Between readings and at night, thesensor was exposed to an air equilibrated saline drip. During the secondthird and fourth days, when hourly measurements were made, there was anegative mm drift between successive readings. This drift was reversedduring overnight saline drip. On the first day, the readings were lowbecause the negative mm drift had already occurred during the sensorcalibration process.

Careful inspection of the fluorescent intensities between samplesindicated that a residual concentration of CO₂ in the sensor betweensample draws. This residual CO₂ partial pressure was estimated to beabout 0.5 mmHg and went away after the first three hours of overnightsaline drip. This was due to some reservoiring of residual CO₂ in thetubing and sensor cassette.

Using a species predominance model (described below in Example 10) todetermine the thermodynamic state of the internal aqueous compartment asa function of CO₂ partial pressure, we calculated that the aqueous phasepH was always below 8.3 during the daytime (even between measurementperiods), and above 9.3 at night, while the osmolarity did not changeappreciably during this period. This led us to conclude that thenegative mm drift was the result of pH dependent migration of acidand/or amine impurities in and out of the internal aqueous phase.Furthermore, these species were present within the sensor and were notintroduced from the medium being sensed. The negative mm drift wasassociated with the reversible migration of acid and amine impuritiesbetween the aqueous phase and the silicone phase.

There are several methods by which one may reduce this drift. First, onemay effectively limit the amount of drift by reducing the driving forcewhich causes the drift (i.e., the pH change in the internal aqueouscompartment). This was accomplished by repeated sampling of the sensorin the 5 volume % CO₂ sparged phosphate buffer. In such a case, theresidual pCO₂ was never below 0.5 mm Hg and thus the sensor is alwaysbelow pH 8.3. The sensor pH was only changed by one unit when laterexposed to the 5 volume % equilibrated medium (rather than by 2 units aswould be the case for a sensor which starts at a pCO₂ of 0.25 mm Hg).Thus the amount of partitioning was reduced by a factor of 10. A similarresult followed, if after exposure to the 5 volume percent medium thesensor was stored in medium having a pCO₂ of 0.5 mm Hg.

Drift free performance can also be achieved by employing a mixedcarbonate/phosphate buffer in the internal aqueous compartment to keepthe aqueous phase pH below 8.3 during the overnight saline drip period.

Notably, the black teflon overcoat can trap the migratable species inthe sensor, making it more difficult to remove these migratableimpurities by washing.

Example 9

We discovered that the two principle partitioning species in the sensorsof Example 8 were acid (e.g., sodium acetate present from themanufacture of the hydroxyethylcellulose and other acids from thecrosslinker) and base (e.g., ammonia from the manufacture of thesilicone base polymer and the fumed silica filler) impurities. We havedeveloped a species predominance model to determine the thermodynamicstate of the internal aqueous compartment as a function of CO₂ partialpressure, buffer concentration, and the level of migratable impurities.Using this model as a guide, we can modify the buffer composition of theaqueous phase to minimize acid induced drift, amine induced drift, orboth. By deconvolving acid and amine induced drift in this way, we areable to track down the source of impurities and arrive at a cleanersensor composition. Also, we can formulate sensors with alternate buffercompositions and indicators that are more stable to drift in thepresence of modest levels of impurities.

Model for the Aqueous Phase

The internal aqueous compartment is comprised of a mixture of buffersand additional salts. In the simplest case, there are the acid and baseforms of the indicator dye, the various forms of carbonate buffer inequilibrium with a partial pressure of CO₂, and sodium chloride added toprovide a desired osmolarity. The following representative equilibriaapply.

    CO.sub.2 (gas)⃡CO.sub.2 (dissolved)

    CO.sub.2 (dissolved)⃡HCO.sub.3.sup.- +H.sup.+

    HCO.sub.3.sup.- ⃡CO.sub.3.sup.2- +H.sup.+

    HPTS.sup.3- (acid)⃡HPTS.sup.4- (base)+H.sup.+

    H.sub.2 O⃡OH.sup.- +H.sup.+

When impurities are present (acetic acid and ammonia shown), thefollowing representative equilibria additionally apply.

    CH.sub.3 OOH⃡CH.sub.3 OO.sup.- +H.sup.+

    NH.sub.4.sup.+ ⃡NH.sub.3 +H.sup.+

Acid dissociation constants relate the concentration of each bufferspecies to the total buffer concentration and to the concentration ofhydronium ions, as shown below for CO₂. ##EQU1## where: "s" is the molarconcentration of dissolved CO₂ per unit partial pressure of CO₂ and ascorrected for temperature and ionic strength effects according toexperimental data.

We used experimental data in a lookup table to correct the CO₂solubility "s" for temperature and ionic strength effects. We alsocorrected the acid dissociation constants for temperature and ionicstrength effects using methods discussed later.

Our model iteratively converges on a self-consistent solution to thegeneral equation for charge balance (equation 1), at a particular CO₂partial pressure and at a fixed temperature.

    [Na.sup.+ ]+[H.sup.+ ]--[OH.sup.31 ]--[Cl.sup.- ]+Σz.sub.i C.sub.i (H.sup.+)=0                                               eq.1

where:

z_(i) is the ionic charge of species i, and

C_(i) (H⁺) is the pH dependent concentration of ionic buffer species.The ionic strength (I) is initially estimated from the concentration ofexcess sodium chloride concentration. From this, the CO₂ solubility andacid dissociation constants are estimated. These are used to estimatethe concentrations of the acid and base forms of the buffers as afunction of H⁺ concentration. The H⁺ concentration is then varied untilequation 1 is satisfied. The preliminary solution for [H⁺ ] correspondsto a preliminary concentration of each buffer species. With thisinformation, a new estimate of the ionic strength is made. In turn, theCO₂ solubility and acid dissociation constants are updated. The processis repeated until a self consistent solution for [H⁺ ] and I areobtained. This iterative process is repeated at each new partialpressure of CO₂ to determine sensor response characteristics.

Acid dissociation constants, K_(a), are determined as follows. First,the acid dissociation constants are estimated at zero ionic strength andabsolute temperature T using experimental data, summarized in functionalform in equation 2.

    -log(K.sub.a)=A.sub.1 /T-A.sub.2 +A.sub.3 T                eq.2

Next, the ionic strength effects are determined. Traditional estimatesof apparent acid dissociation constants (K_(a) ') require assumptionsabout the activity coefficients of individual ions, which cannot bemeasured experimentally. By assuming an excess of sodium chloride, wehave recast the problem in terms of experimentally measurableparameters, namely the activity coefficients of ion pairs, f±. Sodiumion is the dominate counterion for negatively charged acids and bases;chloride ion is the dominant counterion for positively charged acids andbases. For charge neutral and anionic acids, the result is shown inequation 3, where J is the charge on the deprotonated form of thebuffer. For cationic acids, the result is shown in equation 4. ##EQU2##Instead of using a lookup table, the ionic strength and temperaturedependence of the experimental activity coefficients for each bufferspecies have been fit to the Debye-Huckel-Onsanger equation 5, where "a"and "b" are adjustable parameter related to ionic radius and specificionic interactions, respectively. z₁ and z₂ are the ionic charges. A andB are thermodynamic parameters which depend on temperature anddielectric constant of the medium. ##EQU3## To achieve a convergentsolution to equation 1, each updated estimate of ionic strength isplugged into a family of equations of the form of equation 5 todetermine activity coefficients for each ion pair. These activities areplugged into a family of equations of the form of equations 3 or 4 toupdate the estimates of the apparent acid dissociation constants foreach buffer.

To estimate the sensitivity of a sensor to changes in CO₂ we calculatethe pH of the aqueous phase at several CO₂ partial pressures. Theconcentration of the basic form of HPTS is calculated and fit to amathematical relationship or calibration equation. Alternatively, weestimate the differential change in concentration of the basic form ofthe indicator (I₁ -I₂ /I₁) at two CO₂ partial pressures (2.8% and 8.4%CO₂) which bracket the range of interest (for clinical monitoring ofblood gases in this case). For the case of HPTS, (I₁ -I₂)/I₁ correspondsto the change in fluorescence of a sensor when excited at 460 mm. Forthe purpose of this invention, it is recognized by those skilled in theart that (I₁ -I₂)/I₁ can be related to measurements of other indicatorswhere, for example, the acidic form of the indicator is measured, theratio of acid and base forms of the indicator is monitored, the phaseshift of the total emission is measured, etc. Our modeling programmatches experimental measurements with the following accuracies(pH±0.02, osmolarities±0.1%, and HPTS predominance±1%).

Model for Sensor Drift

While we can estimate the amount of an impurity in a sensor by chemicalanalysis, the partitioning coefficients (K_(p)) of migratable impuritiesbetween the aqueous and silicone phases is not generally known.Therefore, the absolute concentrations of these impurities in theaqueous compartment is not known with certainty.

In our model, we assume an initial concentration of migratable impurityin the internal aqueous compartment at a particular pH. We also assumethat the silicone is an infinite reservoir for accumulation of chargeneutral acids or amines. With these assumptions, we estimate the changein concentration of aqueous impurities as a function of CO₂ partialpressure. This provides relative estimates of drift. The followingdiscussion demonstrates these concepts for acetate induced drift.

At thermodynamic equilibrium, the concentration of a charge neutral acidis dictated by a pH dependent acid dissociation equilibrium and asilicone dependent acid partitioning equilibrium.

    [CH.sub.3 OOH].sub.aq =[H.sup.+ ][CH.sub.3 COO.sup.- ]/K.sub.a 'eq.6

    [CH.sub.3 OOH].sub.aq =K.sub.p [CH.sub.3 OOH].sub.silicone eq. 7

We define a thermodynamic reference point for the aqueous phase, forexample 3.3 mF acetate at pH 9.0, for which the equations for chargebalance (eq 1) and partitioning (eq. 7) are satisfied. This defines avalue for the quantity K_(p) [CH₃ OOH]_(silicone) on the right side ofequation 7. Next we allow the contaminated sensor to equilibrate withair. Acetic acid is allowed to migrate between the silicone and theaqueous phase until equations 1 and 7 are satisfied at this new pH. Atthis point, we determine a calibration relationship for the airequilibrated sensor. Next we elevate the CO₂ partial pressure, to 6% forexample. The pH drops substantially and the concentration of HPTS(base)is noted. Acetic acid is then allowed to migrate from the aqueous phaseinto the silicone until equations for charge balance (eq 1) andpartitioning (eq. 7) are again satisfied. In the process, the bufferingcapacity of the aqueous compartment changes, the pH rises to a newvalue, and the concentration of HPTS(base) increases. The CO₂conditioning drift at 6% CO₂ is expressed in % mm drift (to equilibriumand not merely for a three hour period as described in FIG. 3b), usingthe calibration equation for the air equilibrated sensor.

Results from Modeling

FIG. 10a shows the calculated response for several drift free CO₂ sensorformulations made with HPTS indicator and sodium bicarbonate buffer. Theformal concentrations of Na₃ HPTS and NaHCO₃ are shown along the x and yaxes, respectively. FIG. 10e shows the calculated response for severaldrift free CO₂ sensor formulations made with 10 mM HPTS indicator,sodium bicarbonate buffer and a mixture of dibasic and monobasicphosphate buffer. The sodium chloride concentration is maintained at 128mM throughout this example. Migratible impurities are absent. Contoursin FIGS. 10a and 10e represent aqueous formulations with the same valueof the ratio (I₁ -I₂ /I₁). For clinical applications, values of (I₁ -I₂/I₁) in the range of 0.2 to 0.5, and more preferably in the range of0.30 to 0.40. This corresponds to formal sodium bicarbonateconcentrations between 5 and 40 mM and more preferably between 10 and 20mM.

Importantly, the pKa of the HPTS dye is dependent on ionic strength andis near 7.1 in FIG. 10a. By increasing the pKa of the indicator, thedesired values of (I₁ -I₂ /I₁) are achieved at higher formalconcentrations of sodium bicarbonate. For example, by replacing the HPTSwith carboxy SNARF-X (pKa=7.9), the desired values of (I₁ -I₂ /I₁) areachieved using sodium bicarbonate concentrations between 25 to 200 mMand more preferably between 55 to 100 mM.

For sensors containing only amine impurities, CO₂ conditioning drift isreduced by using high concentrations of NaHCO₃. The desired values of(I₁ -I₂ /I₁) are achieved by choosing an indicator with acorrespondingly high pKa. FIG. 10b shows the % mm drift (when an airequilibrated sensor is moved into a medium equilibrated with 6 volume %CO₂) calculated when the NaHCO₃ based sensors of FIG. 10a arecontaminated with ammonia to a thermodynamic reference point of 3.0 mFaqueous ammonium at pH 7.0. FIG. 10f shows the % mm drift (when an airequilibrated sensor is moved into a medium equilibrated with 6 volume %CO₂) calculated when the mixed phosphate based sensors of FIG. 10e arecontaminated with ammonia to a thermodynamic reference point of 3.0 mFaqueous ammonium at pH 7.0. For both FIGS. 10b and 10f the % mm drift isreduced from over 25% to less than 6% by increasing the NaHCO₃concentration from 10 mM to greater than 40 mM. CO₂ conditioning driftis reduced because the operating range of the sensor is more basic andmore of the amine impurity remains in the silicone.

Table 9a shows the decline in amine induced drift as the concentrationof sodium bicarbonate is increased from 15 to 32 to 78 mM. A desiredsensitivity of (I₁ -I₂)/I₁ =0.32-0.36 is achieved through appropriatechoice of indicator pKa, in this case HPTS, Dextran SNAFL-2, and carboxySNARF-X, respectively. Attachment of SNAFL-2 to the dextran not onlymodifies the pKa but also prevents possible migration of the dye fromthe aqueous phase into the silicone. Note that for all three of thebicarbonate based sensors in Table 9a, the change in pH (air→6% CO₂)remains near 2 pH units. The decline in amine induced drift at higherbicarbonate concentrations is principally due to the increase in theoperating pH, not a decrease in ΔpH. The higher operating pH drives moreof the amine into the silicone at all CO₂ partial pressures therebyreducing the amount of amine involved in partitioning.

For sensors containing only acid impurities, (CO₂ conditioning drift isreduced by using lower concentrations of NaHCO₃. The desired values of(I₁ -I₂ /I₁) are achieved by choosing an indicator with acorrespondingly low pKa. In this case, drift free performance isachieved by using buffer compositions that are prepared from lowconcentrations of NaHCO₃ (5-15 mM) and elevated concentrations of theindicator and/or a phosphate buffer, introduced as a 50:50 mixture ofNaH₂ PO₄ and Na₂ HPO₄. FIG. 10c shows the % mm drift (when an airequilibrated sensor is moved into a medium equilibrated with 6 volume %CO₂) calculated when the NaHCO₃ based sensors of FIG. 10a arecontaminated with acetic acid to a thermodynamic reference point of 3.3mF aqueous acetate at pH 9.0. The % mm drift is reduced from over 20% toless than 6% by decreasing the NaHCO₃ concentration from 20 mM to lessthan 10 mM at an HPTS concentration of 10 mM. However, at a lowerconcentration of HPTS (e.g., 2 mM) improved performance cannot beachieved. Drift is reduced at the 10 mM HPTS concentration because theHPTS serves as a buffer to limit the pH change on going from airequilibrated medium to 6% CO₂. Added phosphate buffer can have the sameeffect in reducing acid induced drift. FIG. 10g shows the % mm drift(when an air equilibrated sensor is moved into a medium equilibratedwith 6 volume % CO₂) calculated when the mixed phosphate based sensorsof FIG. 10e are contaminated with acetic acid to a thermodynamicreference point of 3.3 mF aqueous acetate at pH 9.0. The % mm drift isreduced below 6% by decreasing the NaHCO₃ concentration below 15 mM atan HPTS concentration of 10 mM and a mixed phosphate bufferconcentration of 10 mM.

FIG. 10d shows the % mm drift (when an air equilibrated sensor is movedinto a medium equilibrated with 6 volume % CO₂) calculated for sensorshaving buffer compositions prepared from 15 mM NaHCO₃ and variousamounts of HPTS and a 50:50 mixture of NaH₂ PO₄ and Na₂ PO₄ andcontaminated with acetic acid to a thermodynamic reference point of 3.3mF aqueous acetate at pH 9.0.

In the absence of phosphate buffer, drift is reduced from 20% to lessthan 4% as the HPTS concentration is increased from 5 to 15 mM. Also, atan HPTS concentration of 10 mM, the drift decreases from 13% to lessthan 4% as the mixed phosphate concentration is increased from 0 to 10mM. More generally, as the ratio ##EQU4## goes from 0.33 to 1.0 thedrift decreases from 20% to less than 4%.

Table 9b shows the extent of acid induced drift for HPTS based sensordesigned to have sensitivities between (I₁ -I₂)/I₁ =0.32-0.36. Allsensors have aqueous phases prepared from 15 mM NaHCO₃ and variousamounts of HPTS indicator and the mixed phosphate buffer. For sensorsprepared with 5 mM HPTS indicator, the drift decreases from 20% to lessthan 4% as the mixed phosphate concentration is increased from 0 to 25mM. Note that the reduction in drift is due to a decrease in ΔpH (air→6%) for the more highly buffered sensors.

When there is a mixture of both acid and amine impurities, drift issignificant at all concentrations of NaHCO₃. At low NaHCO₃, drift isdominated by amines. At high NaHCO₃, drift is dominated by acids. If theamount of the acid and amine impurities is not too large, a buffer withintermediate properties can be used to reduce the drift. In this case,drift-free performance is achieved by using buffer compositions that areprepared from higher concentrations of NaHCO₃ (to reduce amine induceddrift) and elevated concentrations of indicator and/or phosphate buffer(to reduce acid induced drift). Here we use a carbonate/phosphate mixedbuffer system to substantially reduce the pH change between air and 6%CO₂.

FIG. 10e shows the calculated sensitivity (I₁ -I₂ /I₁) for CO₂ sensorformulations prepared with 10 mM HPTS indicator and various amounts ofNaHCO₃ and mixed phosphate buffer. Again, the sodium chlorideconcentration is 128 mM. Migratible impurities are absent. Contoursrepresent aqueous formulations with the same value of (I₁ -I₂ /I₁).

In this case, drift free sensors can be prepared using aqueous phasecompositions having NaHCO₃ concentrations greater than 20 mF (andpreferably greater than 40 mF) and having concentrations of HPTS andmixed phosphate buffer which satisfy the condition that ##EQU5## isgreater than 0.6, more preferably greater than 0.8 and most preferablygreater than 1.0. the indicator is chosen such that the pKa providessensitivity of (I₁ -I₂)/I₁ between 0.2 and 0.5 and more preferablybetween 0.3 and 0.4.

For sensors prepared as described in Run #17 of Table 2a (run 5 of Table9c), both acetic acid and ammonia impurities were present. Drift wasgreater than 16%/hr. Table 9c shows that the drift can be reduced byemploying aqueous phase compositions in which the concentration of HPTSis increased and a mixed phosphate buffer is added.

Buffer compositions used in runs 1, 2 and 4 of Table 9c all have thesame susceptibility to amine induced drift and a decreasingsusceptibility to acid induced drift. This indicates that acid induceddrift has been substantially eliminated and that amine induced drift isprincipally responsible for the remaining 4-5% of drift.

                  TABLE 9a                                                        ______________________________________                                        Indicator                                                                              pKa     [NaHCO.sub.3 ]                                                                          Drift                                                                              pH.sub.air                                                                          pH.sub.6%                                                                           ΔpH                         ______________________________________                                        HPTS     7.05    15        -26  9.24  7.16  2.08                              Dextran  7.5     32        -5   9.47  7.49  1.98                              SNAFL-2                                                                       Carboxy  7.9     78        -3   9.70  7.86  1.84                              SNARF-X                                                                       ______________________________________                                    

                  TABLE 9b                                                        ______________________________________                                        Phosphate HPTS      Drift  pH.sub.air                                                                           pH.sub.6%                                                                           ΔpH                             ______________________________________                                        0         5         -20    9.11   7.08  2.03                                  0         10        -13    8.85   6.98  1.87                                  0         15         -3    8.15   6.88  1.27                                  5         5         -17    9.01   7.05  1.96                                  15        5          -7    8.63   7.00  1.63                                  25        5          -2    7.76   6.96  0.8                                   ______________________________________                                    

                  TABLE 9c                                                        ______________________________________                                        Run   HPTS    Na.sub.2 CO.sub.3                                                                      Na.sub.3 PO.sub.4                                                                     Na.sub.2 HPO.sub.4                                                                    Drift (5/hr)                           ______________________________________                                        1     10      10       --      --      4.2                                    2     10      --       10      --      4.4                                    3     10      --        5      5       3.7                                    4     10      --       20      --      4.6                                    5     6.36      6.6    --      --      16.5                                   ______________________________________                                    

Example 10

Following the procedure of Example 1, Run 1, a sensor was tested for itsresponse time. A similar sensor, but containing 1.0 mM HPTS, 8 mM Na₂CO₃ and 144 mM CaCl, was also prepared and tested. The sensor of Example1, Run 1 had a response time of 120 seconds when moved from air to 4.9%CO₂. The other sensor had a response time of 42 seconds when moved fromair to 4.9% CO₂. Both sensors were substantially drift free.

Various modifications and alterations of this invention will be apparentto those skilled in the art without departing from the scope and spiritof this invention, and it should be understood that this invention isnot limited to the illustrative embodiments set forth herein.

What is claimed is:
 1. A sensing composition for sensing a carbondioxide analyte, comprising:an aqueous first phase comprising a pHsensitive indicator component in a buffer solution, wherein said buffersolution comprises less than 100 mM of a bicarbonate ion, wherein theconcentration of said pH sensitive indicator component in said sensingcomposition is effective to provide a signal in response to theconcentration of carbon dioxide in a medium to which said sensingcomposition is exposed, said carbon dioxide being effective to alter thepH of said aqueous phase; and a hydrophobic second phase which ispermeable to said analyte and impermeable to ionized hydrogen, whereinsaid aqueous phase is dispersed within said hydrophobic phase, andwherein said sensing composition is substantially free of partitioningspecies, other than the analyte of interest, which migrate between saidphases in response to a change in pH in said aqueous phase or betweenthe sensing composition and the medium and which migration substantiallyaffects said concentration dependent signal.
 2. A sensing compositionaccording to claim 1, wherein said sensing composition, immediatelyafter being equilibrated in a first medium having a pCO₂ of 0.25 mmHg,provides a measurement of CO₂ partial pressure which drifts less than12% over a three hour period when moved from said first medium to asecond medium having a pCO₂ of 45.6 mmHg.
 3. A sensing compositionaccording to claim 1, wherein said sensing composition, immediatelyafter being equilibrated in a first medium having a pCO₂ of 0.25 mmHg,provides a measurement of CO₂ partial pressure which drifts less than 6%over a three hour period when moved from said first medium to a secondmedium having a pCO₂ of 45.6 mmHg.
 4. A sensing composition according toclaim 1, wherein said sensing composition, immediately after beingequilibrated in a first medium having a pCO₂ of 0.25 mmHg, provides ameasurement of CO₂ partial pressure which drifts less than 3% over athree hour period when moved from said first medium to a second mediumhaving a pCO₂ of 45.6 mmHg.
 5. A sensing composition according to claim1, wherein said sensing composition, after being stored for 24 hours ina first medium having a pCO₂ of 0.25 mmHg, provides a measurement of CO₂partial pressure when moved to a second medium having a pCO₂ of 45.6mmHg which differs by less than 12% compared to a measurement taken insaid second medium before the 24 hour storage period.
 6. A sensingcomposition according to claim 1, wherein said sensing composition,after being stored for 24 hours in a first medium having a pCO₂ of 0.25mmHg, provides a measurement of CO₂ partial pressure when moved to asecond medium having a pCO₂ of 45.6 mmHg which differs by less than 6%compared to a measurement taken in said second medium before the 24 hourstorage period.
 7. A sensing composition according to claim 1, whereinsaid sensing composition, immediately after being equilibrated in afirst medium having a pCO₂ of 0.25 mmHg, provides a measurement of CO₂partial pressure which drifts less than 6% over a three hour period whenmoved from said first medium to a second medium having a pCO₂ of 45.6mmHg, and wherein said sensing composition, after being stored for 24hours in said a first medium, provides a measurement of CO₂ partialpressure when moved to said second medium which differs by less than 6%compared to a measurement taken in said second medium before the 24 hourstorage period.
 8. A sensing composition according to claim 1, whereinsaid aqueous phase and said hydrophobic phase form an emulsoid.
 9. Asensing composition according to claim 10, wherein said emulsoidcomprises micro-compartments of said aqueous phase having an averagesize of up to 10 μm.
 10. A sensing composition according to claim 1,wherein said aqueous phase and said hydrophobic phase form an emulsoidwhich prior to being cured formed a stable emulsion.
 11. A sensingcomposition according to claim 1, wherein said pH sensitive indicatorcomponent is present in said aqueous phase in at least an effectiveamount up to a concentration of 15 mM and is selected from the groupconsisting of 9-amino-6-chloro-2-methoxyacridine;2',7'-bis-(2-carboxyethyl)-5-(and -6)-carboxyfluorescein;2',7'-bis-(2-carboxyethyl)-5-(and -6)-carboxyfluorescein, acetoxymethylester; 2',7'-bis-(2-carboxyethyl)-5-(and -6)-carboxyfluorescein,acetoxymethyl ester; 5-(and -6)-carboxy-2',7'-dichlorofluorescein;5-(and -6)-carboxy-2',7'-dichlorofluorescein diacetate; 5-(and-6)-carboxy-4',5'-dimethylfluorescein; 5-(and-6)-carboxy-4',5'-dimethylfluorescein diacetate; 5-carboxyfluorescein;6-carboxyfluorescein; 5-(and -6)carboxyfluorescein; 5-carboxyfluoresceindiacetate; 6-carboxyfluorescein diacetate; 5-carboxyfluoresceindiacetate, acetoxymethyl ester; 5-(and -6)-carboxyfluorescein diacetate;5-(and -6)-carboxynaphthofluorescein; 5-(and-6)-carboxynaphthofluorescein diacetate; 5'(and6')-succinimidylester-3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one; 5'(and6')-succinimidylester-9-chloro-3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one;5'(and 6')-carboxy-3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one;5'(and6')-carboxy-3,10-diacetoxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one;5'(and6')-carboxy-9-chloro-3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one;5'(and6')-carboxy-9-chloro-3,10-diacetoxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one;5'(and 6')-carboxy-10-dimethylamino-3-hydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one;3-acetoxy-5'-acetoxymethoxycarbonyl-10-dimethylaminospiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one;5'(and6')-carboxy-10-diethylamino-3-hydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one;3-acetoxy-5'-acetoxymethoxycarbonyl-10-diethylamine-3-hydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one;5'(and6')-carboxy-10-ethylamino-3-hydroxy-9-methyl-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one;5'(and6')-carboxy-3-hydroxytetrahydroquinolizino[1,9-hi]spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one;5-chloromethylfluorescein diacetate; 4-chloromethyl-7-hydroxycoumarin;4-[2-chloro-6-(ethylamino)-7-methyl-3-oxo-3H-xanthen-9-yl]-1,3-benzene-dicarboxylicacid; dextran, 2',7'-bis(2-carboxyethyl)-5(and 6)-carboxy-fluorescein,anionic; dextran,4-[2-chloro-6-(ethylamino)-7-methyl-3-oxo-3H-xanthen-9-yl]-1,3-benzene-dicarboxylicacid, anionic; dextran,4-[2-chloro-6-(ethylamino)-7-methyl-3-oxo-3H-xanthen-9-yl]-1,3-benzene-dicarboxylicacid, anionic, lysine fixable; dextran,4-[2,7-dimethyl-6-(ethylamino)-3-oxo-3H-xanthen-9-yl]-1,3-benzenedicarboxylic acid, anionic; dextran,4-[2,7-dimethyl-6-(ethylamino)-3-oxo-3H-xanthen-9-yl]-1,3-benzenedicarboxylic acid, anionic, lysine fixable; dextran, 7-hydroxycoumarin,neutral; dextran, β-methylumbelliferone, neutral; dextran,9-chloro-3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one,anionic; dextran,10-dimethylamino-3-hydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one,anionic; 1,4-dihydroxyphthalonitrile;4-[2,7-dimethyl-6-(ethylamino)-3-oxo-3H-xanthen-9-yl]-1,3-benzenedicarboxylic acid; fluorescein diacetate;8-hydroxypyrene-1,3,6-trisulfonic acid, trisodium salt;naphthofluorescein; naphthofluorescein diacetate;3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one;and3,10-diacetoxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one,diacetate.
 12. A sensing composition according to claim 3, wherein saidpH sensitive indicator component is present in said aqueous phasebetween a concentration of 2 and 10 mM and is selected from the groupconsisting of 2',7'-bis-(2-carboxyethyl)-5-(and -6)-carboxyfluorescein;5-(and -6)-carboxy-2',7'-dichlorofluorescein; 5-(and-6)-carboxy-4',5'-dimethylfluorescein; carboxy5'(and6')-carboxy-3,10-dihydroxy-spiro[7H-benzo[c]xanthene7,1'(3'H)-isobenzofuran]-3'-one;carboxy5'(and6')-carboxy-9-chloro-3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one;carboxy5'(and6')-carboxy-10-dimethylamino-3-hydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one;carboxy5'(and6')-carboxy-10-diethylamino-3-hydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one;carboxy5'(and6')-carboxy-3-hydroxytetrahydroquinolizino[1,9-hi]spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one; 4-chloromethyl-7-hydroxycoumarin;4-[2-chloro-6-(ethylamino)-7-methyl-3-oxo-3H-xanthen-9-yl]-1,3-benzene-dicarboxylicacid; dextran, 2',7'-bis(2-carboxyethyl)-5(and 6)-carboxy-fluorescein,anionic; dextran,4-[2-chloro-6-(ethylamino)-7-methyl-3-oxo-3H-xanthen-9-yl]-1,3-benzene-dicarboxylicacid, anionic; dextran, 7-hydroxycoumarin, neutral; dextran,β-methylumbelliferone, neutral; dextran,9-chloro-3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one,anionic; dextran,10-dimethylamino-3-hydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one,anionic; 8-hydroxypyrene-1,3,6-trisulfonic acid, trisodium salt; and3,10-dihydroxy-spiro[7H-benzo[c]xanthene-7,1'(3'H)-isobenzofuran]-3'-one.13. A sensing composition according to claim 8, wherein said pHsensitive indicator component has an apparent pKa between 6.3 and 8.5.14. A sensing composition according to claim 13, wherein said aqueousphase further comprises a buffer selected from the group consisting ofphosphate ion buffers and wherein said phosphate ion buffer is presentin an concentration up to 30 mM based on the total aqueous phasecomponents.
 15. A sensing composition according to claim 12, whereinsaid aqueous phase further comprises a emulsification enhancement agentselected from the group consisting of poly(ethylene oxide),poly(ethylene glycol), poly(acrylamide), poly(dimethylacrylamide),poly(vinylmethyl acetomide), poly(styrene sulfonate),poly(vinylalcohol), poly(vinylpyrollidone), poly(hydroxyethylacrylate),poly(hydroxymethylacrylate), poly(acrylic acid), poly(vinyl methylether), dextran, hydroxyethylcellulose, hydroxypropyl cellulose,carboxymethylcellulose, methylcellulose, hydroxypropyl(methylcellulose),hydroxybutyl(methylcellulose), xanthan gum, guar gum, alginic acid, andcarageenen, wherein said agent is present in an concentration between 1and 50 weight percent based on the total aqueous phase components, andwherein said sensor comprises less than 3 mM of acidic or basicpartitioning species, based on the volume of said aqueous phase,throughout the operating range of the sensor.
 16. A sensing compositionaccording to claim 4, wherein said sensor comprises less than 1 mM of anacidic partitioning species, based on the volume of said aqueous phase,throughout the operating range of the sensor.
 17. A sensing compositionaccording to claim 4, wherein said sensor comprises less than 1 mM of abasic partitioning species, based on the volume of said aqueous phase,throughout the operating range of the sensor.
 18. A sensing compositionaccording to claim 4, wherein said hydrophobic phase comprises asilicone polymer or copolymer.
 19. A sensing composition for sensingcarbon dioxide, comprising:an aqueous first phase comprising a pHsensitive indicator component, wherein the concentration of said pHsensitive indicator component in said sensing composition is effectiveto provide a signal in response to the concentration of carbon dioxidein a medium to which said sensing composition is exposed, said carbondioxide being effective to alter the pH of said aqueous phase; and ahydrophobic second phase which is permeable to said analyte andimpermeable to ionized hydrogen, wherein said aqueous phase is dispersedwithin said hydrophobic phase, and wherein said sensing compositioncomprises less than 1 mM of acidic or basic partitioning species, otherthan the analyte of interest, which migrate between said phases inresponse to a change in pH in said aqueous phase or between the sensingcomposition and the medium and which migration substantially affectssaid concentration dependent signal.
 20. A sensing composition accordingto claim 19, wherein said aqueous phase comprises a buffer solutionequivalent in composition to a solution prepared with an indicatorcomponent and up to 100 mM of a bicarbonate salt, and wherein saidsensing composition, immediately after being equilibrated in a firstmedium having a pCO₂ of 0.25 mmHg, provides a measurement of CO₂ partialpressure which drifts less than 3% over a three hour period when movedfrom said first medium to a second medium having a pCO₂ of 45.6 mmHg.21. A sensing composition for sensing carbon dioxide, comprising:anaqueous first phase comprising an effective amount of a pH sensitiveindicator component in a buffer solution, wherein said buffer solutionhas a buffering capacity equivalent to a solution prepared with up to 20mM of said pH sensitive indicator component, between 20 and 100 mM of abicarbonate salt, and up to 50 mM of a 50:50 mix of a monobasic anddibasic phosphates salt, wherein said pH sensitive indicator componentis effective to provide a signal in response to the concentration ofcarbon dioxide in a medium to which said sensing composition is exposed,said carbon dioxide being effective to alter the pH of said aqueousphase; and a hydrophobic second phase which is permeable to said analyteand impermeable to ionized hydrogen, wherein said aqueous phase isdispersed within said hydrophobic phase, wherein said sensingcomposition is substantially free of acidic partitioning species, otherthan the analyte of interest, which migrate between said phases inresponse to a change in pH in said aqueous phase or between the sensingcomposition and the medium and which migration substantially affectssaid concentration dependent signal, wherein said sensing composition,immediately after being equilibrated in a first medium having a pCO₂ of0.25 mmHg, provides a measurement of CO₂ partial pressure which driftsless than 12% over a three hour period when moved from said first mediumto a second medium having a pCO₂ of 45.6 mmHg, and wherein said sensingcomposition has a specific sensitivity between 0.2 and 0.5.
 22. Asensing composition according to claim 21, wherein said sensingcomposition, immediately after being equilibrated in a first mediumhaving a pCO₂ of 0.25 mmHg, provides a measurement of CO₂ partialpressure which drifts less than 6% over a three hour period when movedfrom said first medium to a second medium having a pCO₂ of 45.6 mmHg.23. A sensing composition according to claim 22, wherein said buffersolution is equivalent in composition to a solution prepared with up to20 mM of a pH sensitive indicator component and between 40 and 80 mM ofa bicarbonate salt.
 24. A sensing composition according to claim 21,wherein said sensor comprises less than 1 mM of an acidic partitioningspecies and less than 3 mM of a basic partitioning species, based on thevolume of said aqueous phase, throughout the operating range of thesensor.
 25. A sensing composition according to claim 21, wherein saidindicator component is a salt of HPTS, and said hydrophobic phasecomprises silicone.
 26. A sensing composition for sensing carbondioxide, comprising:an aqueous first phase comprising an effectiveamount of a pH sensitive indicator component in a buffer solution,wherein said buffer solution has a buffering capacity equivalent to asolution prepared with up to 20 mM of said pH sensitive indicatorcomponent, at least 20 mM of a bicarbonate salt, and up to 50 mM of a50:50 mix of a monobasic and dibasic phosphates salt, wherein said pHsensitive indicator component is effective to provide a signal inresponse to the concentration of carbon dioxide in a medium to whichsaid sensing composition is exposed, said carbon dioxide being effectiveto alter the pH of said aqueous phase; and a hydrophobic second phasewhich is permeable to said analyte and impermeable to ionized hydrogen,wherein said aqueous phase is dispersed within said hydrophobic phase,wherein said sensing composition is substantially free of acidicpartitioning species, other than the analyte of interest, which migratebetween said phases in response to a change in pH in said aqueous phaseor between the sensing composition and the medium and which migrationsubstantially affects said concentration dependent signal, wherein saidsensing composition, immediately after being equilibrated in a firstmedium having a pCO₂ of 0.25 mmHg, provides a measurement of CO₂ partialpressure which drifts less than 12% over a three hour period when movedfrom said first medium to a second medium having a pCO₂ of 45.6 mmHg,wherein said sensing composition has a specific sensitivity between 0.2and 0.5, and wherein said aqueous phase does not include the combinationof HPTS and polyvinylpyrolidone.
 27. A sensing composition according toclaim 26, wherein said sensing composition, immediately after beingequilibrated in a first medium having a pCO₂ of 0.25 mmHg, provides ameasurement of CO₂ partial pressure which drifts less than 3% over athree hour period when moved from said first medium to a second mediumhaving a pCO₂ of 45.6 mmHg.
 28. A sensing composition according to claim27, wherein said buffer solution is equivalent in composition to asolution prepared with up to 20 mM of a pH sensitive indicator componentand at least 50 mM of a bicarbonate salt.
 29. A sensing compositionaccording to claim 26, wherein said sensor comprises less than 1 mM ofan acidic partitioning species and less than 3 mM of a basicpartitioning species, based on the volume of said aqueous phase,throughout the operating range of the sensor.
 30. A sensing compositionaccording to claim 26, wherein said indicator component is a salt ofHPTS, and said hydrophobic phase comprises silicone.
 31. A sensingcomposition for sensing carbon dioxide, comprising:an aqueous firstphase comprising a pH sensitive indicator component in a buffersolution, wherein said buffer solution has a buffering capacityequivalent to a solution prepared with said pH sensitive indicatorcomponent and at least one of a bicarbonate salt and a 50:50 mixture ofa monobasic phosphate salt and a dibasic phosphate salt, wherein the sumof the molar concentration of the indicator component and one-half ofthe molar concentration of the mixture of phosphate salts divided by themolar concentration of the bicarbonate salt is at least 0.66, whereinsaid pH sensitive indicator component is effective to provide a signalin response to the concentration of carbon dioxide in a medium to whichsaid sensing composition is exposed, said carbon dioxide being effectiveto alter the pH of said aqueous phase; and a hydrophobic second phasewhich is permeable to said analyte and impermeable to ionized hydrogen,wherein said aqueous phase is dispersed within said hydrophobic phase,wherein said sensing composition is substantially free of basicpartitioning species, other than the analyte of interest, which migratebetween said phases in response to a change in pH in said aqueous phaseor between the sensing composition and the medium and which migrationsubstantially affects said concentration dependent signal, wherein saidsensing composition, immediately after being equilibrated in a firstmedium having a pCO₂ of 0.25 mmHg, provides a measurement of CO₂ partialpressure which drifts less than 12% over a three hour period when movedfrom said first medium to a second medium having a pCO₂ of 45.6 mmHg,and wherein said sensing composition has a specific sensitivity between0.2 and 0.5.
 32. A sensing composition according to claim 31, whereinsaid sensing composition, immediately after being equilibrated in afirst medium having a pCO₂ of 0.25 mmHg, provides a measurement of CO₂partial pressure which drifts less than 6% over a three hour period whenmoved from said first medium to a second medium having a pCO₂ of 45.6mmHg.
 33. A sensing composition according to claim 31, wherein saidsensing composition, immediately after being equilibrated in a firstmedium having a pCO₂ of 0.25 mmHg, provides a measurement of CO₂ partialpressure which drifts less than 3% over a three hour period when movedfrom said first medium to a second medium having a pCO₂ of 45.6 mmHg.34. A sensing composition according to claim 32, wherein said sensorcomprises less than 3 mM of an acidic partitioning species and less than1 mM of a basic partitioning species, based on the volume of saidaqueous phase, throughout the operating range of the sensor.
 35. Asensing composition according to claim 32, wherein said indicatorcomponent is a salt of HPTS, and said hydrophobic phase comprisessilicone, and wherein said sensor comprises less than 1 mM of an acidicpartitioning species and less than 0.5 mM of a basic partitioningspecies, based on the volume of said aqueous phase, throughout theoperating range of the sensor.
 36. A sensing composition for sensingcarbon dioxide, comprising:an aqueous first phase comprising a pHsensitive indicator component in a buffer solution, wherein said buffersolution has a buffering capacity equivalent to a solution prepared withsaid pH sensitive indicator component, at least 15 mM of a bicarbonatesalt, and up to 50 mM of a 50:50 mixture of a monobasic and dibasicphosphate salt, wherein the ratio of the sum of the molar concentrationof the indicator component and one-half of the molar concentration ofthe mixture of phosphate salts divided by the molar concentration of thebicarbonate salt is at least 0.66, wherein said pH sensitive indicatorcomponent is effective to provide a signal in response to theconcentration of carbon dioxide in a medium to which said sensingcomposition is exposed, said carbon dioxide being effective to alter thepH of said aqueous phase; and a hydrophobic second phase which ispermeable to said analyte and impermeable to ionized hydrogen, whereinsaid aqueous phase is dispersed within said hydrophobic phase, whereinsaid sensing composition, immediately after being equilibrated in afirst medium having a pCO₂ of 0.25 mmHg, provides a measurement of CO₂partial pressure which drifts less than 12% over a three hour periodwhen moved from said first medium to a second medium having a pCO₂ of45.6 mmHg, and wherein said sensing composition has a specificsensitivity between 0.2 and 0.5.
 37. A sensing composition according toclaim 36, wherein said buffer solution ratio is at least 0.8.
 38. Asensing composition according to claim 36, wherein said buffer solutionratio is at least 1.0.
 39. A sensing composition according to claim 36,wherein said sensing composition, immediately after being equilibratedin a first medium having a pCO₂ of 0.25 mmHg, provides a measurement ofCO₂ partial pressure which drifts less than 12% over a three hour periodwhen moved from said first medium to a second medium having a pCO₂ of45.6 mmHg.
 40. A sensing composition according to claim 37, wherein saidsensing composition, immediately after being equilibrated in a firstmedium having a pCO₂ of 0.25 mmHg, provides a measurement of CO₂ partialpressure which drifts less than 6% over a three hour period when movedfrom said first medium to a second medium having a pCO₂ of 45.6 mmHg.41. A sensing composition, comprising:an aqueous first phase comprisingan indicator component, wherein said pH sensitive indicator component iseffective to provide a signal in response to the concentration of ananalyte in a medium to which said sensing composition is exposed, saidanalyte being effective to alter the pH of said aqueous phase; and ahydrophobic second phase which is permeable to said analyte andimpermeable to ionized hydrogen, wherein said aqueous phase is containedwithin the pores of an asymmetrical membrane, said membrane having afirst surface having small pore openings and a second surface havinglarge pore openings, and wherein said hydrophobic phase covers saidfirst surface.
 42. A sensing composition according to claim 41, whereinsaid sensing composition, immediately after being equilibrated in afirst medium having a pCO₂ of 0.25 mmHg, provides a measurement of CO₂partial pressure which drifts less than 12% over a three hour periodwhen moved from said first medium to a second medium having a pCO₂ of45.6 mmHg.
 43. A sensing composition for sensing a carbon dioxideanalyte, comprising:an aqueous first phase comprising a pH sensitiveindicator component, a buffer solution containing less than 100 mM of abicarbonate ion, and a hydrogel of a hydrophilic polymer, wherein theconcentration of said pH sensitive indicator component in said sensingcomposition is effective to provide a signal in response to theconcentration of carbon dioxide in a medium to which said sensingcomposition is exposed, said carbon dioxide being effective to alter thepH of said aqueous phase; and a hydrophobic second phase which ispermeable to said analyte and impermeable to ionized hydrogen, whereinsaid sensing composition is substantially free of partitioning species,other than the analyte of interest, which migrate between said phases inresponse to a change in pH in said aqueous phase or between the sensingcomposition and the medium and which migration substantially affectssaid concentration dependent signal.
 44. A sensing composition accordingto claim 43, wherein said hydrogel comprises a polymer formed usingmonomers selected from the group consisting of acrylates andmethacrylates.
 45. A sensing composition according to claim 44, whereinsaid pH sensitive indicator component is covalently attached to saidpolymer.
 46. A sensing composition according to claim 43, wherein saidsensing composition comprises less than 1 mM of acidic partitioningspecies.
 47. A sensing composition according to claim 43, wherein saidsensing composition, immediately after being equilibrated in a firstmedium having a pCO₂ of 0.25 mmHg, provides a measurement of CO₂ partialpressure which drifts less than 12% over a three hour period when movedfrom said first medium to a second medium having a pCO₂ of 45.6 mmHg.48. A sensing composition according to claim 43, wherein said sensingcomposition further comprises an emulsification enhancement agentselected from the group consisting of poly(ethylene oxide),poly(ethylene glycol), poly(acrylamide), poly(dimethylacrylamide),poly(vinylalcohol), poly(vinylmethyl acetamide), poly(vinylpyrollidone),poly(styrene sulfonate), poly(acrylamidomethylproponesulfonic acid),poly(hydroxyethylacrylate), poly(hydroxymethylacrylate), poly(acrylicacid), poly(vinyl methyl ether), hydroxyethylcellulose,hydroxypropylcellulose, carboxymethylcellulose, methylcellulose,hydroxypropyl-(methylcellulose), and hydroxybutyl(methylcellulose).